@article{dezman2023,

title = {A mechatronic leg replica to benchmark human\textendashexoskeleton physical interactions},

author = {Miha De\v{z}man and Stefano Massardi and David Pinto-Fernandez and Victor Grosu and Carlos Rodriguez-Guerrero and Jan Babi\v{c} and Diego Torricelli},

doi = {10.1088/1748-3190/accda8},

issn = {1748-3182},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {Bioinspiration \& Biomimetics},

volume = {18},

issue = {3},

pages = {036009},

abstract = {Evaluating human\textendashexoskeleton interaction typically requires experiments with human subjects, which raises safety issues and entails time-consuming testing procedures. This paper presents a mechatronic replica of a human leg, which was designed to quantify physical interaction dynamics between exoskeletons and human limbs without the need for human testing. In the first part of this work, we present the mechanical, electronic, sensory system and software solutions integrated in our leg replica prototype. In the second part, we used the leg replica to test its interaction with two types of commercially available wearable devices, i.e. an active full leg exoskeleton and a passive knee orthosis. We ran basic test examples to demonstrate the functioning and benchmarking potential of the leg replica to assess the effects of joint misalignments on force transmission. The integrated force sensors embedded in the leg replica detected higher interaction forces in the misaligned scenario in comparison to the aligned one, in both active and passive modalities. The small standard deviation of force measurements across cycles demonstrates the potential of the leg replica as a standard test method for reproducible studies of human-exoskeleton physical interaction.},

keywords = {Exoskeleton Design and Control, Neuromusculoskeletal Modelling, Physical Human Robot Interaction, Robot Design},

pubstate = {published},

tppubtype = {article}

}

@article{Diaz2023b,

title = {Human-in-the-Loop Optimization of Wearable Robotic Devices to Improve Human\textendashRobot Interaction: A Systematic Review},

author = {Mar\'{i}a Alejandra D\'{i}az and Matthias Vo\ss and Arnau Dillen and Bruno Tassignon and Louis Flynn and Joost Geeroms and Romain Meeusen and Tom Verstraten and Jan Babi\v{c} and Philipp Beckerle and Kevin De Pauw},

url = {https://ieeexplore.ieee.org/document/9994612/},

doi = {10.1109/TCYB.2022.3224895},

issn = {2168-2267},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {IEEE Transactions on Cybernetics},

volume = {53},

issue = {12},

pages = {7483-7496},

abstract = {This article presents a systematic review on wear- able robotic devices that use human-in-the-loop optimization (HILO) strategies to improve human\textendashrobot interaction. A total of 46 HILO studies were identified and divided into upper and lower limb robotic devices. The main aspects from HILO were iden- tified, reviewed, and classified in four areas: 1) human\textendashmachine systems; 2) optimization methods; 3) control strategies; and 4) experimental protocols. A variety of objective functions (phys- iological, biomechanical, and subjective), optimization strategies, and optimized control parameters configurations used in differ- ent control strategies are presented and analyzed. An overview of experimental protocols is provided, including metrics, tasks, and conditions tested. Moreover, the relevance given to train- ing or adaptation periods was explored. We outline an HILO framework that includes current wearable robots, optimization strategies, objective functions, control strategies, and experimen- tal protocols. We conclude by highlighting current research gaps and defining future directions to improve the development of advanced HILO strategies in upper and lower limb wearable robots.},

keywords = {Exoskeleton Design and Control, Human Performance Augmentation, Human-in-the-Loop Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Massardi2023,

title = {Relevance of hazards in exoskeleton applications: a survey-based enquiry},

author = {Stefano Massardi and David Pinto-Fernandez and Jan Babi\v{c} and Miha De\v{z}man and Andrej Tro\v{s}t and Victor Grosu and Dirk Lefeber and Carlos Rodriguez and Jule Bessler and Leendert Schaake and Gerdienke Prange-Lasonder and Jan F. Veneman and Diego Torricelli},

doi = {10.1186/s12984-023-01191-y},

issn = {1743-0003},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {Journal of NeuroEngineering and Rehabilitation},

volume = {20},

issue = {1},

pages = {68},

abstract = {Exoskeletons are becoming the reference technology for assistance and augmentation of human motor functions in a wide range of application domains. Unfortunately, the exponential growth of this sector has not been accompanied by a rigorous risk assessment (RA) process, which is necessary to identify the major aspects concerning the safety and impact of this new technology on humans. This situation may seriously hamper the market uptake of new products. This paper presents the results of a survey that was circulated to understand how hazards are considered by exoskeleton users, from research and industry perspectives. Our analysis aimed to identify the perceived occurrence and the impact of a sample of generic hazards, as well as to collect suggestions and general opinions from the respondents that can serve as a reference for more targeted RA. Our results identified a list of relevant hazards for exoskeletons. Among them, misalignments and unintended device motion were perceived as key aspects for exoskeletons’ safety. This survey aims to represent a first attempt in recording overall feedback from the community and contribute to future RAs and the identification of better mitigation strategies in the field.},

keywords = {Ergonomy, Exoskeleton Design and Control, Human Performance Augmentation},

pubstate = {published},

tppubtype = {article}

}

@article{Kunavar2023,

title = {Explicit learning based on reward prediction error facilitates agile motor adaptations},

author = {Tjasa Kunavar and Xiaoxiao Cheng and David W. Franklin and Etienne Burdet and Jan Babi\v{c}},

doi = {10.1371/journal.pone.0295274},

issn = {1932-6203},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {PLOS ONE},

volume = {18},

issue = {12},

pages = {e0295274},

abstract = {Error based motor learning can be driven by both sensory prediction error and reward prediction error. Learning based on sensory prediction error is termed sensorimotor adaptation, while learning based on reward prediction error is termed reward learning. To investigate the characteristics and differences between sensorimotor adaptation and reward learning, we adapted a visuomotor paradigm where subjects performed arm movements while presented with either the sensory prediction error, signed end-point error, or binary reward. Before each trial, perturbation indicators in the form of visual cues were presented to inform the subjects of the presence and direction of the perturbation. To analyse the interconnection between sensorimotor adaptation and reward learning, we designed a computational model that distinguishes between the two prediction errors. Our results indicate that subjects adapted to novel perturbations irrespective of the type of prediction error they received during learning, and they converged towards the same movement patterns. Sensorimotor adaptations led to a pronounced aftereffect, while adaptation based on reward consequences produced smaller aftereffects suggesting that reward learning does not alter the internal model to the same degree as sensorimotor adaptation. Even though all subjects had learned to counteract two different perturbations separately, only those who relied on explicit learning using reward prediction error could timely adapt to the randomly changing perturbation. The results from the computational model suggest that sensorimotor and reward learning operate through distinct adaptation processes and that only sensorimotor adaptation changes the internal model, whereas reward learning employs explicit strategies that do not result in aftereffects. Additionally, we demonstrate that when humans learn motor tasks, they utilize both learning processes to successfully adapt to the new environments.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Latella2022,

title = {Analysis of Human Whole-Body Joint Torques During Overhead Work With a Passive Exoskeleton},

author = {Claudia Latella and Yeshasvi Tirupachuri and Luca Tagliapietra and Lorenzo Rapetti and Benjamin Schirrmeister and Jonas Bornmann and Dasa Gorjan and Jernej \v{C}amernik and Pauline Maurice and Lars Fritzsche and Jose Gonzalez-Vargas and Serena Ivaldi and Jan Babi\v{c} and Francesco Nori and Daniele Pucci},

url = {https://ieeexplore.ieee.org/document/9647004/},

doi = {10.1109/THMS.2021.3128892},

issn = {2168-2291},

year  = {2022},

date = {2022-10-01},

urldate = {2022-10-01},

journal = {IEEE Transactions on Human-Machine Systems},

volume = {52},

number = {5},

pages = {1060--1068},

publisher = {IEEE},

abstract = {Overheadwork is classifiedas one of the major risk factors for the onset of shoulder work-related musculoskeletal disorders and muscle fatigue. Upper-limb exoskeletons can be used to assist workers during the execution of industrial overhead tasks to prevent such disorders. Twelve novice participants have been equipped with inertial and force/torque sensors to simultaneously estimate the whole-body kinematics and the joint torques (i.e., internal articular stress) by means of a probabilistic estimator, while performing an overhead task with a pointing tool. An evaluation has been performed to analyze the effect at the whole-body level by considering the conditions of wearing and not-wearing PAEXO, a passive exoskeleton for upper-limb support during overhead work. Results point out that PAEXO provides a reduction of the whole-body joint effort across the experimental task blocks (from 66% to 86%). Moreover, the analysis along with five different body areas shows that 1) the exoskeleton provides support at the human shoulders by reducing the joint effort at the targeted limbs, and 2) that part of the internal wrenches is intuitively transferred from the upper body to the thighs and legs, which is shown with an increment of the torques at the legs joints. The promising outcomes show that the probabilistic estimation algorithm can be used as a validation metric to quantitatively assess PAEXO performances, paving thus the way for the next challenging milestone, such as the optimization of the human joint torques via adaptive exoskeleton control.},

keywords = {Ergonomy, Exoskeleton Design and Control, Human Performance Augmentation, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Li2022,

title = {A review on interaction control for contact robots through intent detection},

author = {Yanan Li and Aran Sena and Ziwei Wang and Xueyan Xing and Jan Babi\v{c} and Edwin Asseldonk and Etienne Burdet},

url = {https://iopscience.iop.org/article/10.1088/2516-1091/ac8193},

doi = {10.1088/2516-1091/ac8193},

issn = {2516-1091},

year  = {2022},

date = {2022-07-01},

urldate = {2022-07-01},

journal = {Progress in Biomedical Engineering},

volume = {4},

number = {3},

pages = {032004},

abstract = {Interaction control presents opportunities for contact robots physically interacting with their human user, such as assistance targeted to each human user, communication of goals to enable effective teamwork, and task-directed motion resistance in physical training and rehabilitation contexts. Here we review the burgeoning field of interaction control in the control theory and machine learning communities, by analysing the exchange of haptic information between the robot and its human user, and how they share the task effort. We first review the estimation and learning methods to predict the human user intent with the large uncertainty, variability and noise and limited observation of human motion. Based on this motion intent core, typical interaction control strategies are described using a homotopy of shared control parameters. Recent methods of haptic communication and game theory are then presented to consider the co-adaptation of human and robot control and yield versatile interactive control as observed between humans. Finally, the limitations of the presented state of the art are discussed and directions for future research are outlined.},

keywords = {Human-in-the-Loop Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Takahashi2022,

title = {Human Stiffness Perception and Learning in Interacting With Compliant Environments},

author = {Chie Takahashi and Morteza Azad and Vijaykumar Rajasekaran and Jan Babi\v{c} and Michael Mistry},

url = {https://www.frontiersin.org/articles/10.3389/fnins.2022.841901/full},

doi = {10.3389/fnins.2022.841901},

issn = {1662-453X},

year  = {2022},

date = {2022-06-01},

urldate = {2022-06-01},

journal = {Frontiers in Neuroscience},

volume = {16},

number = {June},

pages = {1--13},

abstract = {Humans are capable of adjusting their posture stably when interacting with a compliant surface. Their whole-body motion can be modulated in order to respond to the environment and reach to a stable state. In perceiving an uncertain external force, humans repetitively push it and learn how to produce a stable state. Research in human motor control has led to the hypothesis that the central nervous system integrates an internal model with sensory feedback in order to generate accurate movements. However, how the brain understands external force through exploration movements, and how humans accurately estimate a force from their experience of the force, is yet to be fully understood. To address these questions, we tested human behaviour in different stiffness profiles even though the force at the goal was the same. We generated one linear and two non-linear stiffness profiles, which required the same force at the target but different forces half-way to the target; we then measured the differences in the learning performance at the target and the differences in perception at the half-way point. Human subjects learned the stiffness profile through repetitive movements in reaching the target, and then indicated their estimation of half of the target value (position and force separately). This experimental design enabled us to probe how perception of the force experienced in different profiles affects the participants' estimations. We observed that the early parts of the learning curves were different for the three stiffness profiles. Secondly, the position estimates were accurate independent of the stiffness profile. The estimation in position was most likely influenced by the external environment rather than the profile itself. Interestingly, although visual information about the target had a large influence, we observed significant differences in accuracy of force estimation according to the stiffness profile.},

keywords = {Compliance and Impedance Control, Human Motor Control, Neuromusculoskeletal Modelling, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Eiken2022,

title = {Effects of vision on energy expenditure and kinematics during level walking},

author = {Ola Eiken and Igor B. Mekjavi\'{c} and Jan Babi\v{c} and Ulf Danielsson and Magnus Hallberg and Stylianos N. Kounalakis},

url = {https://link.springer.com/10.1007/s00421-022-04914-6},

doi = {10.1007/s00421-022-04914-6},

issn = {1439-6319},

year  = {2022},

date = {2022-05-01},

urldate = {2022-05-01},

journal = {European Journal of Applied Physiology},

volume = {122},

number = {5},

pages = {1231--1237},

abstract = {Purpose We have previously observed substantially higher oxygen uptake in soldiers walking on terrain at night than when performing the same walk in bright daylight. The aims of the present study were to investigate the influence of vision on mechanical efficiency during slow, horizontal, constant-speed walking, and to determine whether any vision influence is modified by load carriage. Methods Each subject (n = 15) walked (3.3 km/h) for 10 min on a treadmill in four different conditions: (1) full vision, no carried load, (2) no vision, no carried load, (3) full vision with a 25.5-kg rucksack, (4) no vision with a 25.5-kg rucksack. Results Oxygen uptake was 0.94 ± 0.12 l/min in condition (1), 1.15 ± 0.20 l/min in (2), 1.15 ± 0.12 l/min in (3) and 1.35 ± 0.19 l/min in (4). Thus, lack of vision increased oxygen uptake by about 19%. Analyses of movement pattern, by use of optical markers attached to the limbs and torso, revealed considerably shorter step length (12 and 10%) in the no vision (2 and 4) than full vision conditions (1 and 3). No vision conditions (2 and 4) increased step width by 6 and 6%, and increased vertical foot clearance by 20 and 16% compared to full vision conditions (1 and 3). Conclusion The results suggest that vision has a marked influence on mechanical efficiency even during entrained, repetitive movements performed on an obstacle-free horizontal surface under highly predictable conditions.},

keywords = {Kinematics, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{RannaudMonany2022,

title = {Motor imagery helps updating internal models during microgravity exposure},

author = {D. Rannaud Monany and M. Barbiero and F. Lebon and Jan Babi\v{c} and G. Blohm and D. Nozaki and O. White},

url = {https://journals.physiology.org/doi/10.1152/jn.00214.2021},

doi = {10.1152/jn.00214.2021},

issn = {0022-3077},

year  = {2022},

date = {2022-02-01},

urldate = {2022-02-01},

journal = {Journal of Neurophysiology},

volume = {127},

number = {2},

pages = {434--443},

abstract = {Gravity strongly affects the way movements are performed. How internal models process this information to adapt behavior to novel contexts is still unknown. The microgravity environment itself does not provide enough information to optimally adjust the period of natural arm swinging movements to microgravity. However, motor imagery of the task while immersed in microgravity was sufficient to update internal models. These results show that actually executing a task is not necessary to update graviception.},

keywords = {Human Motor Control, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Gorjan2022,

title = {Inter-Individual Variability in Postural Control During External Center of Mass Stabilization},

author = {Da\v{s}a Gorjan and Nejc \v{S}arabon and Jan Babi\v{c}},

url = {https://www.frontiersin.org/articles/10.3389/fphys.2021.722732/full},

doi = {10.3389/fphys.2021.722732},

issn = {1664-042X},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

journal = {Frontiers in Physiology},

volume = {12},

number = {January},

abstract = {Understanding the relation between the motion of the center of mass (COM) and the center of pressure (COP) is important to understand the underlying mechanisms of maintaining body equilibrium. One way to investigate this is to stabilize COM by fixing the joints of the human and looking at the corresponding COP reactions. However, this approach constrains the natural motion of the human. To avoid this shortcoming, we stabilized COM without constraining the joint movements by using an external stabilization method based on inverted cart-pendulum system. Interestingly, this method only stabilized COM of a subgroup of participants and had a destabilizing effect for others which implies significant variability in inter-individual postural control. The aim of this work was to investigate the underlying causes of inter-individual variability by studying the postural parameters of quiet standing before the external stabilization. Eighteen volunteers took part in the experiment where they were standing on an actuated cart for 335 s. In the middle of this period we stabilized their COM in anteroposterior direction for 105 s. To stabilize the COM, we controlled the position of the cart using a double proportional\textendashintegral\textendashderivative controller. We recorded COM position throughout the experiment, calculated its velocity, amplitude, and frequency during the quiet standing before the stabilization, and used these parameters as features in hierarchical clustering method. Clustering solution revealed that postural parameters of quiet standing before the stabilization cannot explain the inter-individual variability of postural responses during the external COM stabilization. COM was successfully stabilized for a group of participants but had a destabilizing effect on the others, showing a variability in individual postural control which cannot be explained by postural parameters of quiet-stance.},

keywords = {Human Motor Control, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2021,

title = {Challenges and solutions for application and wider adoption of wearable robots},

author = {Jan Babi\v{c} and Matteo Laffranchi and Federico Tessari and Tom Verstraten and Domen Novak and Nejc \v{S}arabon and Barkan Ugurlu and Luka Peternel and Diego Torricelli and Jan F. Veneman},

url = {https://www.cambridge.org/core/product/identifier/S263171762100013X/type/journal_article},

doi = {10.1017/wtc.2021.13},

issn = {2631-7176},

year  = {2021},

date = {2021-11-01},

urldate = {2021-11-01},

journal = {Wearable Technologies},

volume = {2},

pages = {e14},

abstract = {The science and technology of wearable robots are steadily advancing, and the use of such robots in our everyday life appears to be within reach. Nevertheless, widespread adoption of wearable robots should not be taken for granted, especially since many recent attempts to bring them to real-life applications resulted in mixed outcomes. The aim of this article is to address the current challenges that are limiting the application and wider adoption of wearable robots that are typically worn over the human body. We categorized the challenges into mechanical layout, actuation, sensing, body interface, control, human\textendashrobot interfacing and coadaptation, and benchmarking. For each category, we discuss specific challenges and the rationale for why solving them is important, followed by an overview of relevant recent works. We conclude with an opinion that summarizes possible solutions that could contribute to the wider adoption of wearable robots.},

keywords = {Exoskeleton Design and Control, Human Performance Augmentation},

pubstate = {published},

tppubtype = {article}

}

@article{Kunavar2021,

title = {Effects of Local Gravity Compensation on Motor Control During Altered Environmental Gravity},

author = {Tja\v{s}a Kunavar and Marko Jam\v{s}ek and Marie Barbiero and Gunnar Blohm and Daichi Nozaki and Charalambos Papaxanthis and Olivier White and Jan Babi\v{c}},

url = {https://www.frontiersin.org/articles/10.3389/fncir.2021.750267/full},

doi = {10.3389/fncir.2021.750267},

issn = {1662-5110},

year  = {2021},

date = {2021-10-01},

urldate = {2021-10-01},

journal = {Frontiers in Neural Circuits},

volume = {15},

abstract = {Our sensorimotor control is well adapted to normogravity environment encountered on Earth and any change in gravity significantly disturbs our movement. In order to produce appropriate motor commands for aimed arm movements such as pointing or reaching, environmental changes have to be taken into account. This adaptation is crucial when performing successful movements during microgravity and hypergravity conditions. To mitigate the effects of changing gravitational levels, such as the changed movement duration and decreased accuracy, we explored the possible beneficial effects of gravity compensation on movement. Local gravity compensation was achieved using a motorized robotic device capable of applying precise forces to the subject's wrist that generated a normogravity equivalent torque at the shoulder joint during periods of microgravity and hypergravity. The efficiency of the local gravity compensation was assessed with an experiment in which participants performed a series of pointing movements toward the target on a screen during a parabolic flight. We compared movement duration, accuracy, movement trajectory, and muscle activations of movements during periods of microgravity and hypergravity with conditions when local gravity compensation was provided. The use of local gravity compensation at the arm mitigated the changes in movement duration, accuracy, and muscle activity. Our results suggest that the use of such an assistive device helps with movements during unfamiliar environmental gravity.},

keywords = {Exoskeleton Design and Control, Human Motor Control, Human Performance Augmentation, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Jamsek2021,

title = {Predictive Exoskeleton Control for Arm-Motion Augmentation Based on Probabilistic Movement Primitives Combined With a Flow Controller},

author = {Marko Jam\v{s}ek and Tja\v{s}a Kunavar and Urban Bobek and Elmar Rueckert and Jan Babi\v{c}},

url = {https://ieeexplore.ieee.org/document/9387088/},

doi = {10.1109/LRA.2021.3068892},

issn = {2377-3766},

year  = {2021},

date = {2021-07-01},

urldate = {2021-07-01},

journal = {IEEE Robotics and Automation Letters},

volume = {6},

number = {3},

pages = {4417--4424},

abstract = {There are many work-related repetitive tasks where the application of exoskeletons could significantly reduce the physical effort by assisting the user in moving the arms towards the desired location in space. To make such controlmore user acceptable, the controller should be able to predict the motion of the user and act accordingly. This letter presents an exoskeleton control method that utilizes probabilistic movement primitives to generate predictions of user movements in real-time. These predictions are used in a flow controller, which represents a novel velocity-field-based exoskeleton control approach to provide assistance to the user in a predictive way. We evaluated our approach with a haptic robot, where a group of twelve participants had to perform movements towards different target locations in the frontal plane. We tested whether we could generalize the predictions for new and unknown target locations whilst providing assistance to the user without changing their kinematic parameters. The evaluation showed that we could accurately predict user movement intentions while at the same time significantly decrease the overall physical effort exerted by the participants to achieve the task.},

keywords = {Exoskeleton Design and Control, Force Control, Human Performance Augmentation, Optimal Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2021,

title = {A Human-Robot Collaboration Framework for Improving Ergonomics During Dexterous Operation of Power Tools},

author = {Wansoo Kim and Luka Peternel and Marta Lorenzini and Jan Babi\v{c} and Arash Ajoudani},

url = {https://linkinghub.elsevier.com/retrieve/pii/S0736584520302945},

doi = {10.1016/j.rcim.2020.102084},

issn = {07365845},

year  = {2021},

date = {2021-04-01},

journal = {Robotics and Computer-Integrated Manufacturing},

volume = {68},

pages = {102084},

abstract = {In this work, we present a novel control approach to human-robot collaboration that takes into account ergonomic aspects of the human co-worker during power tool operations. The method is primarily based on estimating and reducing the overloading torques in the human joints that are induced by the manipulated external load. The human overloading joint torques are estimated and monitored using a whole-body dynamic state model. The appropriate robot motion that brings the human into the suitable ergonomic working configuration is obtained by an optimisation method that minimises the overloading joint torques. The proposed optimisation process includes several constraints, such as the human arm muscular manipulability and safety of the collaborative task, to achieve a task-relevant optimised configuration. We validated the proposed method by a user study that involved a human-robot collaboration task, where the subjects operated a polishing machine on a part that was brought to them by the collaborative robot. A statistical analysis of ten subjects as an experimental evaluation of the proposed control framework is provided to demonstrate the potential of the proposed control framework in enabling ergonomic and task-optimised human-robot collaboration.},

keywords = {Ergonomy, Human Performance Augmentation, Human-in-the-Loop Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Kozinc2021,

title = {Human pressure tolerance and effects of different padding materials with implications for development of exoskeletons and similar devices},

author = {\v{Z}iga Kozinc and Jan Babi\v{c} and Nejc \v{S}arabon},

doi = {10.1016/j.apergo.2021.103379},

issn = {18729126},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Applied Ergonomics},

volume = {93},

abstract = {In this study, we assessed pressure tolerance in 16 healthy participants at the thigh, chest, and pelvic area, using different surfaces (1 cm2, 20 cm2 and different components, used in exoskeleton design), and the effects of different padding materials. Our results showed substantial variability in pressure tolerance among the participants, as well as lower pressure tolerance in females. Regarding the force applied with the exoskeleton components, male participants had higher discomfort threshold (230.3 ± 44.9 N compared to females (116.1 ± 24.6 N) in the chest area. For the applications with 20 cm2 surface, the males also showed higher pain threshold at the thigh (89.3 ± 41.8 N vs. 34.6 ± 27.2 N) and the pelvis (97.6 ± 37.0 N vs. 56.1 ± 29.5 N). All padding materials increased pressure tolerance for 10\textendash38% (p \< 0.001), but little differences between materials were observed.},

keywords = {Ergonomy, Exoskeleton Design and Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Fonda2021,

title = {The medial‐lateral pedal force component correlates with q‐angle during steady‐state cycling at different workloads and cadences},

author = {Borut Fonda and Jan Babi\v{c} and Nejc \v{S}arabon},

doi = {10.3390/app11031004},

issn = {20763417},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Applied Sciences (Switzerland)},

volume = {11},

number = {3},

pages = {1--8},

abstract = {Leg movement during cycling is constrained to the pedal/crank path and predominantly occurs in a sagittal plane. Medial‐lateral force (FML) applied to the pedals is considered as a waste and does not contribute to the pedaling. The aim of this study was to examine the changes in FML across different cadences and workloads, and to examine the correlation with lateral knee movement (Q‐angle). Twenty‐two cyclists completed six trials at three workloads (2, 2.5 and 3 W/kg) and three cadences (75, 85, 95 rpm). Forces were recorded from the force pedal mounted to the left side. Absolute and normalized (to the peak total force) FML were compared across conditions and crosscorrelation with Q‐angle was calculated. Absolute FML was significantly different across cadences and workloads (p \< 0.05) with higher absolute FML at higher cadence. There was no significant difference in normalized FML across the three cadences. There was a significant decrease in normalized FML ($sim$10 N) at higher workloads. Statistically significant correlations were found between the FML and Q‐angle (R = 0.70\textendash0.77). The results demonstrate the link between the FML and Q‐ angle in healthy pain‐free cyclists during stationary cycling. It has also been observed that smaller normalized magnitude of the FML is present when the force effectiveness is increased.},

keywords = {Ergonomy, Human Performance Augmentation, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{Kozinc2021a,

title = {Comparison of subjective responses of low back pain patients and asymptomatic controls to use of spinal exoskeleton during simple load lifting tasks: A pilot study},

author = {\v{Z}iga Kozinc and Jan Babi\v{c} and Nejc \v{S}arabon},

doi = {10.3390/ijerph18010161},

issn = {16604601},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {International Journal of Environmental Research and Public Health},

volume = {18},

number = {1},

pages = {1--9},

abstract = {Spinal exoskeletons have been suggested as an approach for the prevention and rehabilitation of occupational low back pain (LBP). While the state-of-the-art exoskeletons were shown to substantially unload the back, user acceptance is still limited. Perceived discomfort and restriction of freedom of movement are commonly reported. In this pilot study, we explored the differences in subjective responses and user impressions to using passive spinal exoskeleton during a set of simple lifting tasks between LBP patients (n = 12) and asymptomatic individuals (n = 10). Visual analog scales (0\textendash10) were used for all assessments. Overall, the results showed mostly similar responses or slightly more positive responses to the exoskeleton from LBP patients. Most notably, the LBP patients reported a statistically significant (p = 0.048) higher willingness to use the device daily (5.36 ± 4.05) compared to the control group (2.00 ± 1.85) and also gave the device a higher overall grade (6.58 ± 1.98 vs. 4.30 ± 2.26; p = 0.021). This study has demonstrated that individuals with current LBP responded more favorably to the use of the spinal exoskeleton for simple lifting tasks. This implies that current exoskeletons could be appropriate for LBP rehabilitation, but not preventions, as pain-free individuals are less willing to use such devices. Future studies should explore whether different exoskeleton designs could be more appropriate for people with no LBP issues.},

keywords = {Ergonomy, Exoskeleton Design and Control, Human Performance Augmentation, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Fritzsche2021,

title = {Assessing the efficiency of exoskeletons in physical strain reduction by biomechanical simulation with AnyBody Modeling System},

author = {Lars Fritzsche and Pavel E. Galibarov and Christian Gartner and Jonas Bornmann and Michael Damsgaard and Rudolf Wall and Benjamin Schirrmeister and Jose Gonzalez-Vargas and Daniele Pucci and Pauline Maurice and Serena Ivaldi and Jan Babi\v{c}},

doi = {10.1017/wtc.2021.5},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Wearable Technologies},

volume = {2},

abstract = {IntroductionRecently, many industrial exoskeletons for supporting workers in heavy physical tasks have been developed. However, the efficiency of exoskeletons with regard to physical strain reduction has not been fully proved, yet. Several laboratory and field studies have been conducted, but still more data, that cannot be obtained solely by behavioral experiments, are needed to investigate effects on the human body.MethodsThis paper presents an approach to extend laboratory and field research with biomechanical simulations using the AnyBody Modeling System. Based on a dataset recorded in a laboratory experiment with 12 participants using the exoskeleton Paexo Shoulder in an overhead task, the same situation was reproduced in a virtual environment and analyzed with biomechanical simulation.ResultsSimulation results indicate that the exoskeleton substantially reduces muscle activity and joint reaction forces in relevant body areas. Deltoid muscle activity and glenohumeral joint forces in the shoulder were decreased between 54 and 87%. Simultanously, no increases of muscle activity and forces in other body areas were observed.DiscussionThis study demonstrates how a simulation framework could be used to evaluate changes in internal body loads as a result of wearing exoskeletons. Biomechanical simulation results widely agree with experimental measurements in the previous laboratory experiment and supplement such by providing an insight into effects on the human musculoskeletal system. They confirm that Paexo Shoulder is an effective device to reduce physical strain in overhead tasks. The framework can be extended with further parameters, allowing investigations for product design and evaluation.},

keywords = {Ergonomy, Exoskeleton Design and Control, Human Performance Augmentation, Neuromusculoskeletal Modelling, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Gorjan2021,

title = {Induced stabilization of center of mass decreases variability of center of pressure regardless of visual or tactile information},

author = {Da\v{s}a Gorjan and Angelina Bellicha and Jernej \v{C}amernik and Wael Bachta and Jan Babi\v{c}},

doi = {10.1016/j.jbiomech.2020.110199},

issn = {18732380},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Journal of Biomechanics},

volume = {117},

abstract = {Traditional theories claim that center of pressure (COP) is oscillating to minimize the center of mass (COM) movements, contrary to exploratory theories which propose that COP oscillates to increase sensory information flow from the environment. The aim of this work was to better understand the underlying postural control mechanisms, specifically the interplay of COP oscillations and sensory information flow on keeping the COM stable. Eighteen volunteers took part of the experiment divided into three parts based on sensory conditions: eyes opened, eyes closed and eyes closed with lightly touching a fixed object with one finger. Throughout each part the participants had to quietly stand for 335 s. In the middle of each part, we stabilized their COM for 105 s using a robotized waist-pull system. We recorded whole-body kinematics, COP oscillations, electromyographic activity of soleus and tibialis anterior muscles and the force applied by the finger during light touch conditions. The variability of COP significantly decreased when the COM was stabilized in all sensory conditions. The interaction between sensory condition and stabilization was also significant with different decline of COP variability between quiet standing and stabilization part in all three different sensory conditions. Ankle and knee angle variability decreased significantly while the hip angle variability did not. Our findings suggest that COP is not moving to explore the environment, but to attenuate oscillations of the COM. However, possible functional aspect of movement variability to keep the COM stable still remains.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Sever2021,

title = {Postural responses to sudden horizontal perturbations in tai chi practitioners},

author = {Jernej Sever and Jan Babi\v{c} and \v{Z}iga Kozinc and Nejc \v{S}arabon},

doi = {10.3390/ijerph18052692},

issn = {16604601},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {International Journal of Environmental Research and Public Health},

volume = {18},

number = {5},

pages = {1--12},

abstract = {Tai Chi has been shown to elicit numerous positive effects on health and well-being. In this study, we examined reactive postural control after sudden unloading horizontal perturbations, which resembled situations encountered during Tai Chi. The study involved 20 participants, 10 in the Tai Chi group (age: 37.4 ± 7.8 years), who had been regularly training the push-hand technique for at least 7 years, and 10 in the control group, consisting of healthy adults (age: 28.8 ± 5.0). Perturbations were applied at three different positions (hips, shoulders, and arms) via the load-release paradigm. Twenty measurements were carried out for each perturbation position. We measured peak vertical and horizontal forces on the ground (expressed percentage of body mass (%BM)), peak center of pressure displacement and peak horizontal and vertical velocities at the knee, hip and shoulder joints. The Tai Chi group exhibited smaller increases in vertical ground reaction forces when perturbations were applied at the hips (11.5 ± 2.1 vs. 19.6 ± 5.5 %BW; p = 0.002) and the arms (14.1 ± 4.2 vs. 23.2 ± 8.4 %BW; p = 0.005). They also responded with higher horizontal force increase after hip perturbation (16.2 ± 3.2 vs. 13.1 ± 2.5 %BW; p \< 0.001). Similar findings were found when observing various outcomes related to velocities of vertical movement. The Tai Chi group also showed lower speeds of backward movement of the knee (p = 0.005\textendash0.009) after hip (0.49 ± 0.13 vs. 0.85 ± 0.14 m/s; p = 0.005) and arm perturbations (0.97 ± 0.18 vs. 1.71 ± 0.29 m/s; p = 0.005). Center of pressure displacements were similar between groups. Our study demonstrated that engaging in Tai Chi could be beneficial to reactive postural responses after sudden perturbations in a horizontal direction; however, future interventional studies are needed to directly confirm this. Moreover, because of the age difference between the groups, some confounding effects of age cannot be ruled out.},

keywords = {Kinematics, Neuromusculoskeletal Modelling, Postural Balance, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{10.3389/fncir.2021.750176,

title = {Effects of Simulated Microgravity and Hypergravity Conditions on Arm Movements in Normogravity},

author = {Marko Jam\v{s}ek and Tja\v{s}a Kunavar and Gunnar Blohm and Daichi Nozaki and Charalambos Papaxanthis and Olivier White and Jan Babi\v{c}},

url = {https://www.frontiersin.org/article/10.3389/fncir.2021.750176},

doi = {10.3389/fncir.2021.750176},

issn = {1662-5110},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Frontiers in Neural Circuits},

volume = {15},

pages = {150},

abstract = {The human sensorimotor control has evolved in the Earth's environment where all movement is influenced by the gravitational force. Changes in this environmental force can severely impact the performance of arm movements which can be detrimental in completing certain tasks such as piloting or controlling complex vehicles. For this reason, subjects that are required to perform such tasks undergo extensive training procedures in order to minimize the chances of failure. We investigated whether local gravity simulation of altered gravitational conditions on the arm would lead to changes in kinematic parameters comparable to the full-body experience of microgravity and hypergravity onboard a parabolic flight. To see if this would be a feasible approach for on-ground training of arm reaching movements in altered gravity conditions we developed a robotic device that was able to apply forces at the wrist in order to simulate micro- or hypergravity conditions for the arm while subjects performed pointing movements on a touch screen. We analyzed and compared the results of several kinematic parameters along with muscle activity using this system with data of the same subjects being fully exposed to microgravity and hypergravity conditions on a parabolic flight. Both in our simulation and in-flight, we observed a significant increase in movement durations in microgravity conditions and increased velocities in hypergravity for upward movements. Additionally, we noted a reduced accuracy of pointing both in-flight and in our simulation. These promising results suggest, that locally simulated altered gravity can elicit similar changes in some movement characteristics for arm reaching movements. This could potentially be exploited as a means of developing devices such as exoskeletons to aid in training individuals prior to undertaking tasks in changed gravitational conditions.},

keywords = {Exoskeleton Design and Control, Human Motor Control, Human Performance Augmentation, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Trost2021,

title = {A system to measure the human body asymmetries using accelerometers},

author = {Andrej Tro\v{s}t and Marko Jam\v{s}ek and Nejc \v{S}arabon and Jan Babi\v{c}},

url = {https://ev.fe.uni-lj.si/5-2021/Trost.pdf},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Elektrotehniski Vestnik/Electrotechnical Review},

volume = {88},

number = {5},

pages = {267--272},

abstract = {In sports, acute and chronic injuries of athletes are common despite the growing knowledge and interest in the preventive activity in the sports practice. One of the main causes of injuries in athletes are their body asymmetries, which can only be dealt with if properly quantified. Despite the availability of several measuring systems, the majority of them do not allow segmental measurements and those that do are not portable. To solve the issue, a portable measuring system to measure the body asymmetries of the lower part of the human body was designed and manufactured. It measures and stores the data acquired by six accelerometers attached at the foot, tibia and pelvis of the human both legs. The system is compact, lightweight, portable and allowing a local data storage on the measuring device itself and triggering external signal to start the data acquisition. The system is driven by a sophisticated microcontroller that ensures data sampling at a high sampling rate. Results of a preliminary experimental system evaluation and validation are positive and encouraging for its further development and enhancement.},

keywords = {Exoskeleton Design and Control, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{Forman2020b,

title = {Characterizing forearm muscle activity in university-aged males during dynamic radial-ulnar deviation of the wrist using a wrist robot},

author = {Davis A Forman and Garrick N Forman and Edwin Johnatan Avila-Mireles and Maddalena Mugnosso and Jacopo Zenzeri and Bernadette Murphy and Michael W R Holmes},

url = {https://linkinghub.elsevier.com/retrieve/pii/S0021929020303201},

doi = {10.1016/j.jbiomech.2020.109897},

issn = {18732380},

year  = {2020},

date = {2020-07-01},

journal = {Journal of Biomechanics},

volume = {108},

pages = {109897},

abstract = {Functioning as wrist stabilizers, the wrist extensor muscles exhibit higher levels of muscle activity than the flexors in most distal upper-limb tasks. However, this finding has been derived mostly from isometric or wrist flexion-extension protocols, with little consideration for wrist dynamics or radial-ulnar wrist deviations. The purpose of this study was to assess forearm muscle activity during the execution of dynamic wrist radial-ulnar deviation in various forearm orientations (pronation/supination). In 12 healthy university-aged males, surface electromyography (EMG) was recorded from eight muscles of the dominant arm: flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), extensor digitorum (ED), biceps brachii (BB) and triceps brachii (TB). While grasping a handle, participants performed dynamic radial-ulnar deviation using a three-degrees-of-freedom wrist manipulandum. The robotic device applied torque to the handle, in either a radial or ulnar direction, and in one of three forearm postures (30° supinated/neutral/30° pronated). Results indicated that forearm posture influenced the muscles acting upon the hand (FDS/ED), whereas movement phase (concentric-eccentric) and torque direction influenced nearly every muscle. The ECR demonstrated the greatest task-dependency of all forearm muscles, which is possibly reflective of forearm muscle lines of action. Co-contraction ratios were much higher in radial trials than ulnar (Radial: 1.20 ± 0.78, Ulnar: 0.28 ± 0.18, P textless 0.05), suggesting greater FCU and ECU contribution to wrist joint stability in radial-ulnar movement. These findings highlight a greater complexity of wrist extensor function than has previously been reported in isometric work.},

keywords = {Muscle Mechanics, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Jamsek2020,

title = {Gaussian Mixture Models for Control of Quasi-Passive Spinal Exoskeletons},

author = {Marko Jam\v{s}ek and Tadej Petri\v{c} and Jan Babi\v{c}},

url = {https://www.mdpi.com/711802 https://www.mdpi.com/1424-8220/20/9/2705},

doi = {10.3390/s20092705},

issn = {1424-8220},

year  = {2020},

date = {2020-05-01},

urldate = {2020-05-01},

journal = {Sensors},

volume = {20},

number = {9},

pages = {2705},

publisher = {Multidisciplinary Digital Publishing Institute},

abstract = {Research and development of active and passive exoskeletons for preventing work related injuries has steadily increased in the last decade. Recently, new types of quasi-passive designs have been emerging. These exoskeletons use passive viscoelastic elements, such as springs and dampers, to provide support to the user, while using small actuators only to change the level of support or to disengage the passive elements. Control of such devices is still largely unexplored, especially the algorithms that predict the movement of the user, to take maximum advantage of the passive viscoelastic elements. To address this issue, we developed a new control scheme consisting of Gaussian mixture models (GMM) in combination with a state machine controller to identify and classify the movement of the user as early as possible and thus provide a timely control output for the quasi-passive spinal exoskeleton. In a leave-one-out cross-validation procedure, the overall accuracy for providing support to the user was 86 . 72 ± 0 . 86 % (mean ± s.d.) with a sensitivity and specificity of 97 . 46 ± 2 . 09 % and 83 . 15 ± 0 . 85 % respectively. The results of this study indicate that our approach is a promising tool for the control of quasi-passive spinal exoskeletons.},

keywords = {Exoskeleton Design and Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Koopman2020,

title = {Biomechanical evaluation of a new passive back support exoskeleton},

author = {Axel S Koopman and Matthias Naf and Saskia J Baltrusch and Idsart Kingma and Carlos Rodriguez-Guerrero and Jan Babi\v{c} and Michiel P de Looze and Jaap H van Dieen},

url = {https://linkinghub.elsevier.com/retrieve/pii/S0021929020302153},

doi = {10.1016/j.jbiomech.2020.109795},

issn = {00219290},

year  = {2020},

date = {2020-05-01},

urldate = {2020-05-01},

journal = {Journal of Biomechanics},

volume = {105},

pages = {109795},

abstract = {The number one cause of disability in the world is low-back pain, with mechanical loading as one of the major risk factors. To reduce mechanical loading, exoskeletons have been introduced in the workplace. Substantial reductions in back muscle activity were found when using the exoskeleton during static bending and manual materials handling. However, most exoskeletons only have one joint at hip level, resulting in loss of range of motion and shifting of the exoskeleton relative to the body. To address these issues, a new exoskeleton design has been developed and tested. The present study investigated the effect of the SPEXOR passive exoskeleton on compression forces, moments, muscle activity and kinematics during static bending at six hand heights and during lifting of a box of 10 kg from around ankle height using three techniques: Free, Squat and Stoop. For static bending, the exoskeleton reduced the compression force by 13\textendash21 % depending on bending angle. Another effect of the exoskeleton was that participants substantially reduced lumbar flexion. While lifting, the exoskeleton reduced the peak compression force, on average, by 14 %. Lifting technique did not modify the effect of the exoskeleton such that the reduction in compression force was similar. In conclusion, substantial reductions in compression forces were found as a result of the support generated by the exoskeleton and changes in behavior when wearing the exoskeleton. For static bending, lumbar flexion was reduced with the exoskeleton, indicating reduced passive tissue strain. In addition, the reduced peak compression force could reduce the risk of compression induced tissue failure during lifting.},

keywords = {Ergonomy, Exoskeleton Design and Control, Human Performance Augmentation, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Ugurlu2020,

title = {Active Compliance Control Reduces Upper Body Effort in Exoskeleton-Supported Walking},

author = {Barkan Ugurlu and Hironori Oshima and Emre Sariyildiz and Tatsuo Narikiyo and Jan Babi\v{c}},

url = {https://ieeexplore.ieee.org/document/8960446/},

doi = {10.1109/THMS.2019.2961969},

issn = {2168-2291},

year  = {2020},

date = {2020-04-01},

journal = {IEEE Transactions on Human-Machine Systems},

volume = {50},

number = {2},

pages = {144--153},

abstract = {This article presents a locomotion controller for lower limb exoskeletons so as to enable the combined robot and user system to exhibit compliant walking characteristics when interacting with the environment. This is of critical importance to reduce the excessive ground reaction forces during the walking task execution with the aim of improved environmental interaction capabilities. In robot-aided walking support for paraplegics, the user has to actively use his/her upper limbs via crutches to ensure overall balance. By virtue of this requisite, several issues may particularly arise during touchdown instants, e.g., upper body orientation fluctuates, shoulder joints are subject to excessive loading, and arms may need to exert extra forces to counterbalance these effects. In order to reduce the upper body effort via compliant locomotion, the controller is designed to manage the force/position tradeoff by using an admittance controller in each joint. For proof of concept, a series of exoskeleton-aided walking experiments were conducted with the participation of nine healthy volunteers, four of whom additionally walked on an irregular surface for further performance evaluation. The results suggest that the proposed locomotion controller is advantageous over conventional high-gain position tracking in decreasing undesired oscillatory torso motion and total arm force, adequately reducing the required upper body effort.},

keywords = {Compliance and Impedance Control, Exoskeleton Design and Control, Physical Human Robot Interaction, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Rodriguez-Guerrero2020,

title = {Wearable Robots: Taking a Leap From the Lab to the Real World [From the Guest Editors]},

author = {Carlos Rodriguez-Guerrero and Jan Babi\v{c} and Domen Novak},

url = {https://ieeexplore.ieee.org/document/9040485/},

doi = {10.1109/MRA.2020.2967004},

issn = {1070-9932},

year  = {2020},

date = {2020-03-01},

journal = {IEEE Robotics \& Automation Magazine},

volume = {27},

number = {1},

pages = {20--21},

abstract = {The articles present a mix of mechanical design, control algorithms, and human-subject evaluations, thus providing insights into all aspects of wearable robotics for a varied engineering audience.},

keywords = {Exoskeleton Design and Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Baltrusch2020,

title = {SPEXOR passive spinal exoskeleton decreases metabolic cost during symmetric repetitive lifting},

author = {Saskia J Baltrusch and Jaap H van Dieen and Axel S Koopman and Matthias Naf and Carlos Rodriguez-Guerrero and Jan Babi\v{c} and Han Houdijk},

url = {http://link.springer.com/10.1007/s00421-019-04284-6},

doi = {10.1007/s00421-019-04284-6},

issn = {1439-6319},

year  = {2020},

date = {2020-02-01},

journal = {European Journal of Applied Physiology},

volume = {120},

number = {2},

pages = {401--412},

abstract = {Besides mechanical loading of the back, physiological strain is an important risk factor for low-back pain. Recently a passive exoskeleton (SPEXOR) has been developed to reduce loading on the low back. We aimed to assess the effect of this device on metabolic cost of repetitive lifting. To explain potential effects, we assessed kinematics, mechanical joint work, and back muscle activity. We recruited ten male employees, working in the luggage handling department of an airline company and having ample experience with lifting tasks at work. Metabolic cost, kinematics, mechanical joint work and muscle activity were measured during a 5-min repetitive lifting task. Participants had to lift and lower a box of 10 kg from ankle height with and without the exoskeleton. Metabolic cost was significantly reduced by 18 % when wearing the exoskeleton. Kinematics did not change significantly, while muscle activity decreased by up to 16 %. The exoskeleton took over 18\textendash25 % of joint work at the hip and L5S1 joints. However, due to large variation in individual responses, we did not find a significant reduction of joint work around the individual joints. Wearing the SPEXOR exoskeleton decreased metabolic cost and might, therefore, reduce fatigue development and contribute to prevention of low-back pain during repetitive lifting tasks. Reduced metabolic cost can be explained by the exoskeleton substituting part of muscle work at the hip and L5S1 joints and consequently decreasing required back muscle activity.},

keywords = {Ergonomy, Exoskeleton Design and Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Maurice2020b,

title = {Objective and Subjective Effects of a Passive Exoskeleton on Overhead Work},

author = {Pauline Maurice and Jernej \v{C}amernik and Da\v{s}a Gorjan and Benjamin Schirrmeister and Jonas Bornmann and Luca Tagliapietra and Claudia Latella and Daniele Pucci and Lars Fritzsche and Serena Ivaldi and Jan Babi\v{c}},

url = {https://ieeexplore.ieee.org/document/8856265/},

doi = {10.1109/TNSRE.2019.2945368},

issn = {1534-4320},

year  = {2020},

date = {2020-01-01},

journal = {IEEE Transactions on Neural Systems and Rehabilitation Engineering},

volume = {28},

number = {1},

pages = {152--164},

address = {Nancy},

abstract = {Overhead work is a frequent cause of shoulder work-related musculoskeletal disorders. Exoskeletons offering arm support have the potential to reduce shoulder strain, without requiring large scale reorganization of the workspace. Assessment of such systems however requires to take multiple factors into consideration. This paper presents a thorough in-lab assessment of PAEXO, a novel passive exoskeleton for arm support during overhead work. A list of evaluation criteria and associated performance metrics is proposed to cover both objective and subjective effects of the exoskeleton, on the user and on the task being performed. These metrics are measured during a lab study, where 12 participants perform an overhead pointing task with and without the exoskeleton, while their physical, physiological and psychological states are monitored. Results show that using PAEXO reduces shoulder physical strain as well as global physiological strain, without increasing low back strain nor degrading balance. These positive effects are achieved without degrading task performance. Importantly, participants' opinions of PAEXO are positive, in agreement with the objective measures. Thus, PAEXO seems a promising solution to help prevent shoulder injuries and diseases among overhead workers, without negatively impacting productivity.},

keywords = {Ergonomy, Exoskeleton Design and Control, Kinematics, Physical Human Robot Interaction, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Forman2020,

title = {Characterizing forearm muscle activity in young adults during dynamic wrist flexion\textendashextension movement using a wrist robot},

author = {Davis A Forman and Garrick N Forman and Edwin Johnatan Avila-Mireles and Maddalena Mugnosso and Jacopo Zenzeri and Bernadette Murphy and Michael W R Holmes},

url = {https://doi.org/10.1016/j.jbiomech.2020.109908},

doi = {10.1016/j.jbiomech.2020.109908},

issn = {18732380},

year  = {2020},

date = {2020-01-01},

journal = {Journal of Biomechanics},

volume = {108},

pages = {109908},

publisher = {Elsevier Ltd},

abstract = {Current research suggests that the wrist extensor muscles function as the primary stabilizers of the wrist-joint complex. However, most investigations have utilized isometric study designs, with little consideration for wrist dynamics or changes in posture. The purpose of the present study was to assess forearm muscle activity during the execution of dynamic wrist flexion/extension in multiple forearm orientations (pronation/supination). In 12 young adult males, surface electromyography (EMG) was recorded from eight muscles of the dominant arm: flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), extensor digitorum (ED), biceps brachii (BB) and triceps brachii (TB). While grasping a handle, participants performed dynamic wrist flexion/extension using a three-degrees-of-freedom wrist manipulandum. The robotic device applied torque to the handle, in either a flexion or extension direction, and in one of three forearm postures (30° supinated/neutral/30° pronated). Results indicated that forearm posture had minimal influence on forearm muscle activity, but significantly altered the activity of the biceps and triceps brachii. Movement phase (concentric-eccentric) dictated muscle activity in every muscle. Interestingly, muscle activity in the eccentric phase was equal between the two applied handle torques, regardless of whether the muscle acted as the agonist or antagonist. Co-contraction ratios were higher in the flexion conditions (flexion: 2.28 ± 2.04, extension: 0.32 ± 0.27), suggesting significantly greater wrist extensor activity\textendashlikely a contribution to wrist joint stability. This highlights the vulnerability of the wrist extensor muscles to overuse injuries in settings requiring prolonged use of dynamic wrist exertions.},

keywords = {Muscle Mechanics, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Forman2020a,

title = {Investigating the Muscular and Kinematic Responses to Sudden Wrist Perturbations During a Dynamic Tracking Task},

author = {Garrick N Forman and Davis A Forman and Edwin Johnatan Avila-Mireles and Jacopo Zenzeri and Michael W R Holmes},

url = {http://dx.doi.org/10.1038/s41598-020-61117-9},

doi = {10.1038/s41598-020-61117-9},

issn = {20452322},

year  = {2020},

date = {2020-01-01},

journal = {Scientific Reports},

volume = {10},

number = {1},

pages = {1-13},

publisher = {Springer US},

abstract = {Sudden disturbances (perturbations) to the hand and wrist are commonplace in daily activities and workplaces when interacting with tools and the environment. It is important to understand how perturbations influence forearm musculature and task performance when identifying injury mechanisms. The purpose of this work was to evaluate changes in forearm muscle activity and co-contraction caused by wrist perturbations during a dynamic wrist tracking task. Surface electromyography was recorded from eight muscles of the upper-limb. Participants performed trials consisting of 17 repetitions of ±40° of wrist flexion/extension using a robotic device. During trials, participants received radial or ulnar perturbations that were delivered during flexion or extension, and with known or unknown timing. Co-contraction ratios for all muscle pairs showed significantly greater extensor activity across all experimental conditions. Of all antagonistic muscle pairs, the flexor carpi radialis (FCR)-extensor carpi radialis (ECR) muscle pair had the greatest change in co-contraction, producing 1602% greater co-contraction during flexion trials than during extensions trials. Expected perturbations produced greater anticipatory (immediately prior to the perturbation) muscle activity than unexpected, resulting in a 30% decrease in wrist displacement. While improving performance, this increase in anticipatory muscle activity may leave muscles susceptible to early-onset fatigue, which could lead to chronic overuse injuries in the workplace.},

keywords = {Muscle Mechanics, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Galli2020,

title = {Editorial: Embodying Tool Use: From Cognition to Neurorehabilitation},

author = {G. Galli and Y. O. Cakmak and Jan Babi\v{c} and M. Pazzaglia},

doi = {10.3389/fnhum.2020.585670},

issn = {16625161},

year  = {2020},

date = {2020-01-01},

urldate = {2020-01-01},

journal = {Frontiers in Human Neuroscience},

volume = {14},

abstract = {This Research Topic collects an impressive body ofliterature on “Embodying Tool Use.” Overall, the contributions extend and enrich the previous multidisciplinary approach and translational applications. However, despite the significant progress made in our understanding and the real-world relevance, there are boundless directions, endless possibilities, and exciting challenges yet to be explored in future research.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Camernik2020,

title = {Impact of the virtual-height exposure on human psychophysical parameters},

author = {Jernej \v{C}amernik and Sanja Kezi\'{c} and Jan Babi\v{c}},

url = {https://ev.fe.uni-lj.si/5-2020/Camernik.pdf},

year  = {2020},

date = {2020-01-01},

urldate = {2020-01-01},

journal = {Elektrotehniski Vestnik/Electrotechnical Review},

volume = {87},

number = {5},

pages = {267--274},

abstract = {The study evaluates the impact of the virtual height on the human postural control by analyzing the human physiological and psychological responses. Combining the virtual reality and a robotic platform, an environment is created in which a human is seemingly raised to a height of three meters, to affect the human's multi-sensory perception of a movement in the space, causing or enhancing the human's psychophysical responses to an environment at change. A short test is made to monitor the postural control of 20 volunteers during an event of a simultaneous sudden jerk of a robotic platform and a visual change in the height. The data are recorded before and after the event and the results are compared. Using force-plates and an optical system for capturing the volunteer's movement, the movements of forces on the ground and the volunteer's body mass (COP and COM) are observed. Their average values, average power spectrum frequency (MPF) and the root mean square values (RMS) are analyzed. Before and after a virtual rise, the state of the volunteer's current and general perception of anxiety is evaluated with a questionnaire to determine their level of anxiety, electrodermal activity (EDA) and skinsurface temperature. It is shown, that immediately after the volunteer's exposure to a virtual height, their anxiety, fear, skin conductivity and average frequency spectrum of the COP and COM movement increase and their stability, confidence, temperature and RMS values of COP and COM decrease. The volunteer's physiological response to their height perception is also reflected in the change in the mean values of the COP and COM movement in the anterior-posterior direction after they lean backwards, i.e. away from the edge, immediately after lifting. Our study results show that changes in the human emotional and physiological state, as a consequence of a postural threat, and simultaneously also an increase in the human postural control, occur even in humans who are not afraid of the height.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2019,

title = {Target of initial sub-movement in multi-component arm-reaching strategy},

author = {Luka Peternel and Jan Babi\v{c}},

url = {http://www.nature.com/articles/s41598-019-56430-x},

doi = {10.1038/s41598-019-56430-x},

issn = {2045-2322},

year  = {2019},

date = {2019-12-01},

urldate = {2019-12-01},

journal = {Scientific Reports},

volume = {9},

number = {1},

pages = {20101},

abstract = {Goal-directed human reaching often involves multi-component strategy with sub-movements. in general, the initial sub-movement is fast and less precise to bring the limb's endpoint in the vicinity of the target as soon as possible. The final sub-movement then corrects the error accumulated during the previous sub-movement in order to reach the target. We investigate properties of a temporary target of the initial sub-movement. We hypothesise that the peak spatial dispersion of movement trajectories in the axis perpendicular to the movement is in front of the final reaching target, and that it indicates the temporary target of the initial sub-movement. the reasoning is that the dispersion accumulates, due to signal-dependent noise during the initial sub-movement, until the final corrective sub-movement is initiated, which then reduces the dispersion to successfully reach the actual target. We also hypothesise that the reaching movement distance and size of the actual target affect the properties of the temporary target of the initial sub-movement. the increased reaching movement distance increases the magnitude of peak dispersion and moves its location away from the actual target. on the other hand, the increased target size increases the magnitude of peak dispersion and moves its location closer to the actual target.},

keywords = {Human Motor Control, Optimal Control, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Maurice2019a,

title = {Evaluation of PAEXO, a novel passive exoskeleton for overhead work},

author = {Pauline Maurice and Jernej \v{C}amernik and Da\v{s}a Gorjan and Benjamin Schirrmeister and Jonas Bornmann and Luca Tagliapietra and Claudia Latella and Danielle Pucci and Lars Fritzsche and Serena Ivaldi and Jan Babi\v{c}},

url = {https://doi.org/10.1080/10255842.2020.1714977 https://www.tandfonline.com/doi/full/10.1080/10255842.2020.1714977},

doi = {10.1080/10255842.2020.1714977},

issn = {1025-5842},

year  = {2019},

date = {2019-10-01},

journal = {Computer Methods in Biomechanics and Biomedical Engineering},

volume = {22},

number = {sup1},

pages = {S448--S450},

publisher = {Taylor \& Francis},

abstract = {Assessment of physical, physiological, and psycho-logical aspects in the lab study suggest that PAEXO is a promising solution to reduce shoulder WMSDs among overhead workers. An inverse dynamics ana- lysis is being conducted to estimate joint torques from whole-body kinematics and ground reaction force to complement the present assessment. Data collected during field-testing with industrial workers are currently analyzed to evaluate the impact of PAEXO on real end-users.},

keywords = {Ergonomy, Exoskeleton Design and Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2019,

title = {Assistive Arm-Exoskeleton Control Based on Human Muscular Manipulability},

author = {Tadej Petri\v{c} and Luka Peternel and Jun Morimoto and Jan Babi\v{c}},

url = {https://www.frontiersin.org/article/10.3389/fnbot.2019.00030/full https://www.frontiersin.org/articles/10.3389/fnbot.2019.00030/full?\&utm_source=Email_to_authors_\&utm_medium=Email\&utm_content=T1_11.5e1_author\&utm_campaign=Email_publication\&field=\&journalName=Frontiers_in_Neurorobotics\&id=451266},

doi = {10.3389/fnbot.2019.00030},

issn = {1662-5218},

year  = {2019},

date = {2019-05-01},

journal = {Frontiers in Neurorobotics},

volume = {13},

pages = {30},

publisher = {Frontiers},

abstract = {This paper introduces a novel control framework for an arm exoskeleton that takes into account force of the human arm. In contrast to the conventional exoskeleton controllers where the assistance is provided without considering the human arm biomechanical force manipulability properties, we propose a control approach based on the arm muscular manipulability. The proposed control framework essentially reshapes the anisotropic force manipulability into the endpoint force manipulability that is invariant with respect to the direction in the entire workspace of the arm. This allows users of the exoskeleton to perform tasks effectively in the whole range of the workspace, even in areas that are normally unsuitable due to the low force manipulability of the human arm. We evaluated the proposed control framework with real robot experiments where subjects wearing an arm exoskeleton were asked to move a weight between several locations. The results show that the proposed control framework does not affect the normal movement behavior of the users while effectively reduces user effort in the area of low manipulability.Particularly, the proposed approach augments the human arm force manipulability to execute tasks equally well in the entire workspace of the arm.},

keywords = {Compliance and Impedance Control, Human Performance Augmentation, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2019,

title = {SPEXOR: Design and development of passive spinal exoskeletal robot for low back pain prevention and vocational reintegration},

author = {Jan Babi\v{c} and Tadej Petri\v{c} and Katja Mombaur and Idsart Kingma and Jonas Bornmann and Jose Gonzalez-Vargas and Saskia Baltrusch and Nejc \v{S}arabon and Han Houdijk},

url = {http://link.springer.com/10.1007/s42452-019-0266-1},

doi = {10.1007/s42452-019-0266-1},

issn = {2523-3963},

year  = {2019},

date = {2019-03-01},

urldate = {2019-03-01},

journal = {SN Applied Sciences},

volume = {1},

number = {3},

pages = {262},

abstract = {The objective of SPEXOR project is to address low back pain as one of the most appealing health problems of the modern society by creating a body of scientific and technological knowledge in the multidisciplinary areas of biomechanics, robotics, and computer science that will lead to technologies for low back pain prevention. In this paper we provide an overview of the current state-of-art of SPEXOR that the consortium achieved in the first twenty-four months of the project. After introducing the rationale, we describe the biomechanics of low back pain intervention, development of the musculoskeletal stress monitoring for assessment of neuromuscular trunk functions, modeling and optimization of the interaction of spinal exoskeleton with the human body, electromechanical design and development of the passive spinal exoskeleton and its control, and finally the end-user evaluation of the functional effects, usability and satisfaction.},

keywords = {Exoskeleton Design and Control, Robot Design},

pubstate = {published},

tppubtype = {article}

}

@article{Azad2019,

title = {Effects of the weighting matrix on dynamic manipulability of robots},

author = {Morteza Azad and Jan Babi\v{c} and Michael Mistry},

url = {http://link.springer.com/10.1007/s10514-018-09819-y},

doi = {10.1007/s10514-018-09819-y},

year  = {2019},

date = {2019-02-01},

journal = {Autonomous Robots},

volume = {43},

number = {7},

pages = {1867--1879},

abstract = {Dynamic manipulability of robots is a well-known tool to analyze, measure and predict a robot's performance in executing different tasks. This tool provides a graphical representation and a set of metrics as outcomes of a mapping from joint torques to the acceleration space of any point of interest of a robot such as the end-effector or the center of mass. In this paper, we show that the weighting matrix, which is included in the aforementioned mapping, plays a crucial role in the results of the dynamic manipulability analysis. Therefore, finding proper values for this matrix is the key to achieve reliable results. This paper studies the importance of the weighting matrix for dynamic manipulability of robots, which is overlooked in the literature, and suggests two physically meaningful choices for that matrix. We also explain three different metrics, which can be extracted from the graphical representations (i.e. ellipsoids) of the dynamic manipulability analysis. The application of these metrics in measuring a robot's physical ability to accelerate its end-effector in various desired directions is discussed via two illustrative examples.},

keywords = {Human Motor Control, Optimal Control, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{White2019,

title = {The Promise of Stochastic Resonance in Falls Prevention},

author = {Olivier White and Jan Babi\v{c} and Carlos Trenado and Leif Johannsen and Nandu Goswami},

url = {https://www.frontiersin.org/article/10.3389/fphys.2018.01865/full},

doi = {10.3389/fphys.2018.01865},

issn = {1664-042X},

year  = {2019},

date = {2019-01-01},

journal = {Frontiers in Physiology},

volume = {9},

number = {January},

pages = {1--12},

abstract = {Multisensory integration is essential for maintenance of motor and cognitive abilities, thereby ensuring normal function and personal autonomy. Balance control is challenged during senescence or in motor disorders, leading to potential falls. Increased uncertainty in sensory signals is caused by a number of factors including noise, defined as a random and persistent disturbance that reduces the clarity of information. Counter-intuitively, noise can be beneficial in some conditions. Stochastic resonance is a mechanism whereby a particular level of noise actually enhances the response of non-linear systems to weak sensory signals. Here we review the effects of stochastic resonance on sensory modalities and systems directly involved in balance control. We highlight its potential for improving sensorimotor performance as well as cognitive and autonomic functions. These promising results demonstrate that stochastic resonance represents a flexible and non-invasive technique that can be applied to different modalities simultaneously. Finally we point out its benefits for a variety of scenarios including in ambulant elderly, skilled movements, sports and to patients with sensorimotor or autonomic dysfunctions.},

keywords = {Neuromusculoskeletal Modelling, Postural Balance, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Gorjan2019,

title = {Small, movement dependent perturbations substantially alter postural control strategy in healthy young adults},

author = {Da\v{s}a Gorjan and Jan Babi\v{c} and Nejc \v{S}arabon and Zrinka Potocanac},

url = {http://www.sciencedirect.com/science/article/pii/S0021929018307292},

doi = {10.1016/j.jbiomech.2018.09.008},

issn = {18732380},

year  = {2019},

date = {2019-01-01},

journal = {Journal of Biomechanics},

volume = {91},

pages = {1--6},

abstract = {Postural control is commonly investigated by observing responses to perturbations. We developed a perturbation paradigm mimicking self-generated errors in weight shifting, which are a common cause of falling among older adults. Our aim was to determine the effects of this small, but complex, perturbation on postural sway of healthy young adults and evaluate the role of vision and cognition during movement dependent perturbations. Fifteen participants stood hip-width apart with their eyes open, closed and while performing two different cognitive tasks. Participants were continuously perturbed by medial-lateral (ML) support surface translations corresponding to, and hence doubling, their own center of mass sway. We analyzed the standard deviation (SD), root mean square (RMS), range, and mean power frequency (MPF) of center of pressure displacements. ML postural sway increased due to the perturbation (SD p ≤ .001, range p \< .001, RMS p ≤ .001, MPF p \< .001). Cognitive load increased the ML sway range (p = .048). Lack of vision increased ML MPF (p = .001) and anterior-posterior (AP) range (p \< .001), SD (p \< .001), and RMS (p = .001). Significant interaction of vision with the perturbation was found for the ML range (p = .045) and AP SD (p = .018). The perturbation specifically affected ML postural sway. Increased MPF is indicative of a postural control strategy change, which was insufficient for fully controlling the increased sway. Despite being small, this type of perturbation appears to be challenging for young adults.},

keywords = {Kinematics, Muscle Mechanics, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{\v{C}amernik2019b,

title = {Staying on your feet: the effectiveness of posture and handles in counteracting balance perturbation},

author = {Jernej \v{C}amernik and Morteza Azad and Luka Peternel and Zrinka Poto\v{c}anac and Jan Babi\v{c}},

url = {https://doi.org/10.1080/00140139.2018.1559363 https://www.tandfonline.com/doi/full/10.1080/00140139.2018.1559363},

doi = {10.1080/00140139.2018.1559363},

issn = {0014-0139},

year  = {2019},

date = {2019-01-01},

journal = {Ergonomics},

volume = {62},

number = {5},

pages = {657--667},

publisher = {Taylor \& Francis},

abstract = {Stairways, public transport and inclined walkways are often considered as sites with higher likelihood of falls due to a sudden loss of balance. Such sites are usually marked with warning signs, equipped with non-slip surfaces and handles or handrails to avert or decrease this likelihood. Especially, handles are supposed to provide additional support in cases of a sudden loss of balance. However, the mechanisms of using handles for balance at different heights are not yet fully disclosed. We simulated full body perturbations by applying an anterior force to the waist and investigated effectiveness and mechanisms of balance recovery in five different postures: step stance and normal stance with or without holding handles at different heights. Results indicate that both step stance and holding handles at different vertical positions sufficiently assist balance recovery, compared to normal stance. While there was no significant effect of handle in CoM displacement, the shoulder height handle required the lowest handle force, indicating a difference in using the handle. Practitioner summary: To investigate handle use for balance recovery, we perturbed healthy young adults in different standing positions. Even though the use of different handles had a similar effect, the lowest forces were exerted on the shoulder height handle indicating a preferred handle position for balance recovery. Abbreviation: AP: antero-posterior; CNS: Central nervous system; CoM: Center of Mass; CoMmax: Maximal displacement of the center of mass; CoP: Center of pressure; FHmax: Maximal resultant force exerted on the handle; hFHmax: Maximal horizontal force exerted on the handle; vFHmax; Maximal vertical force exerted on the handle; M1-M8: Perturbation force magnitude.},

keywords = {Human Motor Control, Kinematics, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Romeo2019,

title = {Closed-loop Force Control of a Pneumatic Gripper Actuated by Two Pressure Regulators},

author = {Rocco A Romeo and Luca Fiorio and Edwin Johnatan Avila-Mireles and Ferdinando Cannella and Giorgio Metta and Daniele Pucci},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=\&arnumber=8968226},

doi = {10.1109/IROS40897.2019.8968226},

issn = {21530866},

year  = {2019},

date = {2019-01-01},

journal = {IEEE International Conference on Intelligent Robots and Systems},

pages = {7157-7162},

abstract = {Robotic arms can perform grasping actions thanks to their 'dexteorus' part, i.e. the gripper. Among the various categories, nowadays pneumatic grippers became the most employed in industry, as they have low cost and little bulkiness. Despite their simplicity, controlling the force applied by these grippers is not straightforward due to the dependence of such a force on the air pressure in the gripper chambers. As a result, it is still tricky to implement closed-loop force control for pneumatic grippers. This paper intends to deliver a control scheme relying on the force measurement to control pneumatic grippers. The force might be measured through a commercial sensor (e.g. a load cell) and fed back to close the control loop. This includes a calibration which maps the force-pressure relation taking into account both desired force and length of the gripper fingers. The control scheme exploits two different pressure regulators to precisely adjust the air pressure inside the gripper chambers (i.e. opening and closing chambers). To this aim, a quadratic programming algorithm is employed. The control scheme performance revealed to be good: results will be shown in terms of gripper response to sinusoidal and step inputs, along with the pressure-force characterization.},

keywords = {Force Control},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2019,

title = {Study of Patients Self-Training Influence on Peripheral Neuropathies Diseases Diagnosis through D.I.T.A Device},

author = {Edwin Johnatan Avila-Mireles and Haider Abidi and Mariapaola D'Imperio and Massimiliano Scaccia and Paolo Liberini and Darwin G. Caldwell and Ferdinando Cannella},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=\&arnumber=8856599},

doi = {10.1109/EMBC.2019.8856599},

issn = {1557170X},

year  = {2019},

date = {2019-01-01},

journal = {Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS},

pages = {5435-5438},

publisher = {IEEE},

abstract = {In our daily life, the sight and the sense of touch play a fundamental role in objects recognitions. This process is helped by the experience: if a subject has already seen or already touched an object in the past, he will recognize it more easily in the future. Following this assumption, the authors of this paper wanted to investigate if the experience can influence the results of a clinical examination where the subject has an active role. The attention was focused on the peripheral neuropathies diagnosis since they require an accurate assessment of several parameters including the tactile sensitivity trend. In other words, if the tests encompass an active role of the subjects, one of the main uncertainties is the self-training that influences the subject responses. This work focuses on the study of this self-training using the D.I.T.A device (Dynamic Investigation Test-rig on hAptics). Results clearly show a fundamental role of priming during 'haptic modality': expert subjects, previously experienced with the tests, demonstrated better recognition of the encountered stimuli, compared to novices. Moreover, the results show that the maximum difference between the two groups of subjects is in the first part of the test. An ANOVA analysis was carried out to demonstrate that also the errors between the pins-arrays are affected by the priming.},

keywords = {Neuromusculoskeletal Modelling, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Sarabon2019,

title = {Effect of rowing ergometer compliance on biomechanical and physiological indicators during simulated 2,000-metre race},

author = {Nejc \v{S}arabon and \v{Z}iga Kozinc and Jan Babi\v{c} and Goran Markovi\'{c}},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {Journal of Sports Science and Medicine},

volume = {18},

pages = {264--270},

abstract = {This study compared biomechanical characteristics and physiological responses during rowing on three devices: (i) stable ergometer (STE), (ii) transversally compliant ergometer (TCE) and (iii) frontally compliant ergometer (FCE). Eleven young competitive rowers completed a 2000 meter simulated race under each of the ergometer conditions in a randomized order. Stroke rate, average force, power output, velocity and amplitude of the handle and stretcher or seat, heart rate and blood lactate were measured at 500 m intervals. Force and power at the stretcher were significantly lower (p \< 0.03) for TCE, while stroke rate and velocities of the handle and the seat were higher (p \< 0.01). No significant differences were observed between STE and FCE in biomechanical parameters. The lowest rowing performance was observed in FCE (p = 0.007), and was accompanied with the highest average heart rate (p = 0.031). Our findings indicate that in TCE, rowers modified their technique, but were able to maintain physiological strain and performance. In contrast, FCE had no effect on rowing biomechanics, but decreased rowing performance and increased physiological strain. It seems plausible that transversal, but not frontal compliance, elicited a biomechanical technique that might reduce the discrepancy between a rowing ergometer and on-water rowing.},

keywords = {Ergonomy, Neuromusculoskeletal Modelling, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{Teramae2018,

title = {Human-In-The-Loop Control and Task Learning for Pneumatically Actuated Muscle Based Robots},

author = {Tatsuya Teramae and Koji Ishihara and Jan Babi\v{c} and Jun Morimoto and Erhan Oztop},

url = {https://www.frontiersin.org/article/10.3389/fnbot.2018.00071/full},

doi = {10.3389/fnbot.2018.00071},

issn = {1662-5218},

year  = {2018},

date = {2018-11-01},

journal = {Frontiers in Neurorobotics},

volume = {12},

pages = {71},

publisher = {Frontiers},

abstract = {Pneumatically actuated muscles provide a low cost, lightweight and high power-to-weight ratio solution for many robotic applications. In addition, the antagonist pair configuration for robotic arms make it open to biologically inspired control approaches. In spite of these advantages, they have not been widely adopted in human-in-the-loop control and learning applications. In this study, we propose a biologically inspired multimodal human-in-the-loop control system for driving a one degree-of-freedom robot, and realize the task of hammering a nail into a wood block under human control. We analyze the human sensorimotor learning in this system through a set of experiments, and show that effective autonomous hammering skill can be readily obtained through the developed human-robot interface. The results indicate that a human-in-the-loop learning setup with anthropomorphically valid multi-modal human-robot interface leads to fast learning, thus can be used to effectively derive autonomous robot skills for ballistic motor tasks that require continuous modulation of impedance.},

keywords = {Compliance and Impedance Control, Human-in-the-Loop Control, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Romano2018,

title = {The CoDyCo Project Achievements and Beyond: Toward Human Aware Whole-Body Controllers for Physical Human Robot Interaction},

author = {Francesco Romano and Gabriele Nava and Morteza Azad and Jernej Camernik and Stefano Dafarra and Oriane Dermy and Claudia Latella and Maria Lazzaroni and Ryan Lober and Marta Lorenzini and Daniele Pucci and Olivier Sigaud and Silvio Traversaro and Jan Babi\v{c} and Serena Ivaldi and Michael Mistry and Vincent Padois and Francesco Nori},

url = {http://ieeexplore.ieee.org/document/8093992/},

doi = {10.1109/LRA.2017.2768126},

issn = {2377-3766},

year  = {2018},

date = {2018-01-01},

journal = {IEEE Robotics and Automation Letters},

volume = {3},

number = {1},

pages = {516--523},

abstract = {The success of robots in real-world environments is largely dependent on their ability to interact with both humans and said environment.The FP7 EU project CoDyCo focused on the latter of these two challenges by exploiting both rigid and compliant contacts dynamics in the robot control problem. Regarding the former, to properly manage interaction dynamics on the robot control side, an estimation of the human behaviors and intentions is necessary. In this letter, we present the building blocks of such a human-in-the-loop controller, and validate them in both simulation and on the iCub humanoid robot using a human\textendashrobot interaction scenario. In this scenario, a human assists the robot in standing up from being seated on a bench.},

keywords = {Compliance and Impedance Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Sarabon2018,

title = {The Effect of Bed Rest and Hypoxic Environment on Postural Balance and Trunk Automatic (Re)Actions in Young Healthy Males},

author = {Nejc \v{S}arabon and Igor B Mekjavi\'{c} and Ola Eiken and Jan Babi\v{c}},

url = {http://journal.frontiersin.org/article/10.3389/fphys.2018.00027/full},

doi = {10.3389/fphys.2018.00027},

issn = {1664-042X},

year  = {2018},

date = {2018-01-01},

journal = {Frontiers in Physiology},

volume = {9},

pages = {27},

publisher = {Frontiers},

abstract = {Prolonged inactivity, such as bed rest induces several detrimental changes within a short timeframe. Impaired postural balance and responses of trunk muscles to (un)expected perturbations were both shown to be impaired after bed rest. Certain populations (e.g. astronauts) are exposed to hypoxic environment in addition to inactivity, similar to bed rest. While the isolated negative effects of hypoxia on postural balance have been observed before, no study to date has examined the combined effects of hypoxia and bed rest on postural balance or trunk muscle responses. In this study, we examined the effects of 21-day exposure to three conditions: (i) bed rest in hypoxic environment (HBR), (ii) bed rest in normoxic environment (NBR), and (iii) ambulatory hypoxic environment (HAMB). Fourteen healthy male subjects crossed over between conditions in a randomized order, with a 4 month break between conditions to ensure full recovery. Most body sway parameters indicated a similar deterioration of postural balance following both HBR and NBR. Similarly, both anticipatory and reactive responses of the trunk muscles (m. erector spinae and m. multifidus) were impaired after HBR and NBR to a similar degree and mostly unchanged after HAMB. Certain body sway parameters were impaired after HAMB, confirming that hypoxia alone can undermine postural balance. On the other hand, some trunk responses were improved after HAMB. In conclusion, the results of our study confirmed previous findings on negative effects of bed rest, but showed little or no additional effect of hypoxia during bed rest. Physical activity during bed rest is encouraged to preserve neuromuscular functions of the trunk. While the HBR condition in our study resembled conditions during space missions, our results could be relevant to other populations, such as patients with pulmonary diseases exposed to bed rest.},

keywords = {Muscle Mechanics, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2018,

title = {Robotic assembly solution by human-in-the-loop teaching method based on real-time stiffness modulation},

author = {Luka Peternel and Tadej Petri\v{c} and Jan Babi\v{c}},

url = {http://link.springer.com/10.1007/s10514-017-9635-z},

doi = {10.1007/s10514-017-9635-z},

issn = {0929-5593},

year  = {2018},

date = {2018-01-01},

journal = {Autonomous Robots},

volume = {42},

number = {1},

pages = {1--17},

publisher = {Springer US},

abstract = {We propose a novel human-in-the-loop approach for teaching robots how to solve assembly tasks in unpredictable and unstructured environments. In the proposed method the human sensorimotor system is integrated into the robot control loop though a teleoperation setup. The approach combines a 3-DoF end-effector force feedback with an interface for modulation of the robot end-effector stiffness. When operating in unpredictable and unstructured environments, modulation of limb impedance is essential in terms of successful task execution, stability and safety. We developed a novel hand-held stiffness control interface that is controlled by the motion of the human finger. A teaching approach was then used to achieve autonomous robot operation. In the experiments, we analysed and solved two part-assembly tasks: sliding a bolt fitting inside a groove and driving a self-tapping screw into a material of unknown properties. We experimentally compared the proposed method to complementary robot learning methods and analysed the potential benefits of direct stiffness modulation in the force-feedback teleoperation.},

keywords = {Compliance and Impedance Control, Force Control, Human-in-the-Loop Control},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2017b,

title = {Hammering Does Not Fit Fitts' Law},

author = {Tadej Petri\v{c} and Cole S Simpson and Ale\v{s} Ude and Auke J Ijspeert},

url = {http://journal.frontiersin.org/article/10.3389/fncom.2017.00045/full},

doi = {10.3389/fncom.2017.00045},

issn = {1662-5188},

year  = {2017},

date = {2017-01-01},

journal = {Frontiers in Computational Neuroscience},

volume = {11},

number = {May},

pages = {1--12},

abstract = {While movement is essential to human wellbeing, we are still unable to reproduce the deftness and robustness of human movement in automatons or completely restore function to individuals with many types of motor impairment. To better understand how the human nervous system plans and controls movements, neuromechanists employ simple tasks such as upper extremity reaches and isometric force tasks. However, these simple tasks rarely consider impacts and may not capture aspects of motor control that arise from real-world complexity. Here we compared existing models of motor control with the results of a periodic targeted impact task extended from Bernstein's seminal work: hammering a nail into wood. We recorded impact forces and kinematics from 10 subjects hammering at different frequencies and with hammers with different physical properties (mass and face area). We found few statistical differences in most measures between different types of hammer, demonstrating human robustness to minor changes in dynamics. Because human motor control is thought to obey optimality principles, we also developed a feedforward optimal simulation with a neuromechanically inspired cost function that reproduces the experimental data. However, Fitts' Law, which relates movement time to distance traveled and target size, did not match our experimental data. We therefore propose a new model in which the distance moved is a logarithmic function of the time to move that yields better results (R2 ≥ 0.99 compared to R2 ≥ 0.88). These results support the argument that humans control movement in an optimal way, but suggest that Fitts' Law may not generalize to periodic impact tasks.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2017b,

title = {Skill learning and skill transfer mediated by cooperative haptic interaction},

author = {Edwin Johnatan Avila-Mireles and Jacopo Zenzeri and Valentina Squeri and Pietro Morasso and Dalia {De Santis}},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7921690; volume = 25},

doi = {10.1109/TNSRE.2017.2700839},

issn = {15344320},

year  = {2017},

date = {2017-01-01},

journal = {IEEE Transactions on Neural Systems and Rehabilitation Engineering},

number = {7},

pages = {832-843},

abstract = {It is known that physical coupling between two subjects may be advantageous in joint tasks. However, little is known about how two people mutually exchange information to exploit the coupling. Therefore we adopted a reversed, novel perspective to the standard one that focuses on the ability of physically coupled subjects to adapt to cooperative contexts that require negotiating a common plan: we investigated how training in pairs on a novel task affects the development of motor skills of each of the interacting partners. The task involved reaching movements in an unstable dynamic environment using a bilateral non-linear elastic tool that could be used bimanually or dyadically. The main result is that training with an expert leads to the greatest performance in the joint task. However, the performance in the individual test is strongly affected by the initial skill level of the partner. Moreover, practicing with a peer rather than an expert appears to be more advantageous for a naive; and motor skills can be transferred to a bimanual context, after training with an expert, only if the non-expert subject had prior experience of the dynamics of the novel task.},

keywords = {Human Motor Control, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Padois2017,

title = {Whole-body multi-contact motion in humans and humanoids: Advances of the CoDyCo European project},

author = {Vincent Padois and Serena Ivaldi and Jan Babi\v{c} and Michael Mistry and Jan Peters and Francesco Nori},

url = {http://linkinghub.elsevier.com/retrieve/pii/S0921889016304845},

doi = {10.1016/j.robot.2016.08.017},

issn = {09218890},

year  = {2017},

date = {2017-01-01},

journal = {Robotics and Autonomous Systems},

volume = {90},

pages = {97--117},

abstract = {Traditional industrial applications involve robots with limited mobility. Consequently, interaction (e.g. manipulation) was treated separately from whole-body posture (e.g. balancing), assuming the robot firmly connected to the ground. Foreseen applications involve robots with augmented autonomy and physical mobility. Within this novel context, physical interaction influences stability and balance. To allow robots to surpass barriers between interaction and posture control, forthcoming robotic research needs to investigate the principles governing whole-body motion and coordination with contact dynamics. There is a need to investigate the principles of motion and coordination of physical interaction, including the aspects related to unpredictability. Recent developments in compliant actuation and touch sensing allow safe and robust physical interaction from unexpected contact including humans. The next advancement for cognitive robots, however, is the ability not only to cope with unpredictable contact, but also to exploit predictable contact in ways that will assist in goal achievement. Last but not least, theoretical results needs to be validated in real-world scenarios with humanoid robots engaged in whole-body goal-directed tasks. Robots should be capable of exploiting rigid supportive contacts, learning to compensate for compliant contacts, and utilising assistive physical interaction from humans. The work presented in this paper presents state-of-the-art in these domains as well as some recent advances made within the framework of the CoDyCo European project.},

keywords = {Compliance and Impedance Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{GrzinicFrelih2017,

title = {Evaluation of psychological effects on human postural stability},

author = {Nina {Gr\v{z}ini\v{c} Frelih} and Anja Podlesek and Jan Babi\v{c} and Gregor Ger\v{s}ak},

url = {http://linkinghub.elsevier.com/retrieve/pii/S0263224116306844},

doi = {10.1016/j.measurement.2016.11.039},

issn = {02632241},

year  = {2017},

date = {2017-01-01},

journal = {Measurement},

volume = {98},

pages = {186--191},

abstract = {Human postural stability is commonly assessed by a set of posturographic tests during quiet upright standing. Numerous studies extend these tests with cognitive and mental tasks where various physiological and biomechanical sensors are used in combination with a force plate. The aim of our study was to determine whether psychological effects of sensor attachment and the awareness of the subject being measured could influence the posturographic tests performed using the force plate. An experiment was performed where complete data from 51 participants (13 women and 38 men) were obtained in four different conditions. Posturographic tests were performed either with eyes open or eyes closed, and either with biomedical instrumentation sensors attached or not attached. The results indicate that the presence of biomedical instrumentation sensors had a statistically significant impact on the centre of pressure path length and ellipse area as well as on the perceived difficulty of the task and its pleasantness. We conclude that the attachment of sensors on the body of the participants during biomechanical experiments significantly affects the perception of the experimental situation and alters the output of posturographic tests. It is therefore important to appropriately take into account the possible effects of psychological strain (such as the awareness of being measured) in the experimental design and in the interpretation of the results.},

keywords = {Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2017b,

title = {Unifying Speed-Accuracy Trade-Off and Cost-Benefit Trade-Off in Human Reaching Movements},

author = {Luka Peternel and Olivier Sigaud and Jan Babi\v{c}},

url = {http://journal.frontiersin.org/article/10.3389/fnhum.2017.00615/full},

doi = {10.3389/fnhum.2017.00615},

issn = {1662-5161},

year  = {2017},

date = {2017-01-01},

journal = {Frontiers in Human Neuroscience},

volume = {11},

pages = {615},

abstract = {Two basic trade-offs interact while our brain decides how to move our body. First, with the cost-benefit trade-off, the brain trades between the importance of moving faster toward a target that is more rewarding and the increased muscular cost resulting from a faster movement. Second, with the speed-accuracy trade-off, the brain trades between how accurate the movement needs to be and the time it takes to achieve such accuracy. So far, these two trade-offs have been well studied in isolation, despite their obvious interdependence. To overcome this limitation, we propose a new model that is able to simultaneously account for both trade-offs. The model assumes that the central nervous system maximizes the expected utility resulting from the potential reward and the cost over the repetition of many movements, taking into account the probability to miss the target. The resulting model is able to account for both the speed-accuracy and the cost-benefit trade-offs. To validate the proposed hypothesis, we confront the properties of the computational model to data from an experimental study where subjects have to reach for targets by performing arm movements in a horizontal plane. The results qualitatively show that the proposed model successfully accounts for both cost-benefit and speed-accuracy trade-offs.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Potocanac2017,

title = {A robotic system for delivering novel real-time, movement dependent perturbations},

author = {Zrinka Poto\v{c}anac and Rok Goljat and Jan Babi\v{c}},

url = {http://linkinghub.elsevier.com/retrieve/pii/S0966636217308949},

doi = {10.1016/j.gaitpost.2017.08.038},

issn = {09666362},

year  = {2017},

date = {2017-01-01},

journal = {Gait \& Posture},

volume = {58},

pages = {386--389},

abstract = {Perturbations are often used to study movement control and balance, especially in the context of falling. Most often, discrete perturbations defined prior to the experiment are used to mimic external disturbances to balance. However, the largest proportion of falls is due to self-generated errors in weight shifting. Inspired by self-generated weight shifting errors, we created a novel, continuous mediolateral perturbation proportional to subjects' mediolateral center of mass movement with minimal delays. This perturbation was delivered by a robotic platform controlled by a real time Matlab Simulink model using kinematic data from a marker positioned at subjects' L5 as input. Fifteen healthy young adults stood as still as possible atop the robotic platform with their eyes closed. We evaluated the performance of the perturbation in terms of accuracy and delay relative to the input signal by using cross-correlations. The perturbations were accurate (r = −0.984), with delays of 154 ms. Such systematic perturbation significantly affected mediolateral sway, increasing its range (from 5.56 ± 3.72 to 9.58 ± 4.83 mm},

keywords = {Human Motor Control, Postural Balance, Robot Design},

pubstate = {published},

tppubtype = {article}

}

@article{Rueckert2016,

title = {Probabilistic Movement Models Show that Postural Control Precedes and Predicts Volitional Motor Control},

author = {Elmar Rueckert and Jernej \v{C}amernik and Jan Peters and Jan Babi\v{c}},

url = {http://www.nature.com/articles/srep28455},

doi = {10.1038/srep28455},

issn = {2045-2322},

year  = {2016},

date = {2016-09-01},

journal = {Scientific Reports},

volume = {6},

number = {1},

pages = {28455},

publisher = {Nature Publishing Group},

abstract = {Human motor skill learning is driven by the necessity to adapt to new situations. While supportive contacts are essential for many tasks, little is known about their impact on motor learning. To study the effect of contacts an innovative full-body experimental paradigm was established. The task of the subjects was to reach for a distant target while postural stability could only be maintained by establishing an additional supportive hand contact. To examine adaptation, non-trivial postural perturbations of the subjects' support base were systematically introduced. A novel probabilistic trajectory model approach was employed to analyze the correlation between the motions of both arms and the trunk. We found that subjects adapted to the perturbations by establishing target dependent hand contacts. Moreover, we found that the trunk motion adapted significantly faster than the motion of the arms. However, the most striking finding was that observations of the initial phase of the left arm or trunk motion (100-400 ms) were sufficient to faithfully predict the complete movement of the right arm. Overall, our results suggest that the goal-directed arm movements determine the supportive arm motions and that the motion of heavy body parts adapts faster than the light arms.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Optimal Control, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Ivaldi2016,

title = {Special issue on whole-body control of contacts and dynamics for humanoid robots},

author = {Serena Ivaldi and Jan Babi\v{c} and Michael Mistry and Robin Murphy},

url = {http://link.springer.com/10.1007/s10514-016-9545-5},

doi = {10.1007/s10514-016-9545-5},

issn = {0929-5593},

year  = {2016},

date = {2016-03-01},

journal = {Autonomous Robots},

volume = {40},

number = {3},

pages = {425--428},

keywords = {Compliance and Impedance Control, Dynamic Motion, Force Control},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2016b,

title = {Adaptive Control of Exoskeleton Robots for Periodic Assistive Behaviours Based on EMG Feedback Minimisation},

author = {Luka Peternel and Tomoyuki Noda and Tadej Petri\v{c} and Ale\v{s} Ude and Jun Morimoto and Jan Babi\v{c}},

editor = {Dingguo Zhang},

url = {http://dx.plos.org/10.1371/journal.pone.0148942},

doi = {10.1371/journal.pone.0148942},

issn = {1932-6203},

year  = {2016},

date = {2016-02-01},

journal = {PLOS ONE},

volume = {11},

number = {2},

pages = {e0148942},

abstract = {In this paper we propose an exoskeleton control method for adaptive learning of assistive joint torque profiles in periodic tasks. We use human muscle activity as feedback to adapt the assistive joint torque behaviour in a way that the muscle activity is minimised. The user can then relax while the exoskeleton takes over the task execution. If the task is altered and the existing assistive behaviour becomes inadequate, the exoskeleton gradually adapts to the new task execution so that the increased muscle activity caused by the new desired task can be reduced. The advantage of the proposed method is that it does not require biomechanical or dynamical models. Our proposed learning system uses Dynamical Movement Primitives (DMPs) as a trajectory generator and parameters of DMPs are modulated using Locally Weighted Regression. Then, the learning system is combined with adaptive oscillators that determine the phase and frequency of motion according to measured Electromyography (EMG) signals. We tested the method with real robot experiments where subjects wearing an elbow exoskeleton had to move an object of an unknown mass according to a predefined reference motion. We further evaluated the proposed approach on a whole-arm exoskeleton to show that it is able to adaptively derive assistive torques even for multiple-joint motion.},

keywords = {Exoskeleton Design and Control, Human Performance Augmentation, Muscle Mechanics},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2016,

title = {Human motor adaptation in whole body motion},

author = {Jan Babi\v{c} and Erhan Oztop and Mitsuo Kawato},

url = {http://www.nature.com/articles/srep32868},

doi = {10.1038/srep32868},

issn = {2045-2322},

year  = {2016},

date = {2016-01-01},

journal = {Scientific Reports},

volume = {6},

number = {1},

pages = {32868},

publisher = {Nature Publishing Group},

abstract = {The main role of the sensorimotor system of an organism is to increase the survival of the species. Therefore, to understand the adaptation and optimality mechanisms of motor control, it is necessary to study the sensorimotor system in terms of ecological fitness. We designed an experimental paradigm that exposed sensorimotor system to risk of injury. We studied human subjects performing uncon- strained squat-to-stand movements that were systematically subjected to non-trivial perturbation. We found that subjects adapted by actively compensating the perturbations, converging to movements that were different from their normal unperturbed squat-to-stand movements. Furthermore, the adapted movements had clear intrinsic inter-subject differences which could be explained by different adapta- tion strategies employed by the subjects. These results suggest that classical optimality measures of physical energy and task satisfaction should be seen as part of a hierarchical organization of optimality with safety being at the highest level. Therefore, in addition to physical energy and task fulfillment, the risk of injury and other possible costs such as neural computational overhead have to be considered when analyzing human movement.},

keywords = {Human Motor Control, Neuromusculoskeletal Modelling, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Camernik2016,

title = {Holding a Handle for Balance during Continuous Postural Perturbations\textemdashImmediate and Transitionary Effects on Whole Body Posture},

author = {Jernej \v{C}amernik and Zrinka Poto\v{c}anac and Luka Peternel and Jan Babi\v{c}},

url = {http://journal.frontiersin.org/article/10.3389/fnhum.2016.00486},

doi = {10.3389/fnhum.2016.00486},

issn = {1662-5161},

year  = {2016},

date = {2016-01-01},

journal = {Frontiers in Human Neuroscience},

volume = {10},

pages = {486},

publisher = {Frontiers},

abstract = {When balance is exposed to perturbations, hand contacts are often used to assist postural control. We investigated the immediate and the transitionary effects of supportive hand contacts during continuous anteroposterior perturbations of stance by automated waist-pulls. Ten young adults were perturbed for 5 min and required to maintain balance by holding to a stationary, shoulder-high handle and following its removal. Center of pressure (COP) displacement, hip, knee and ankle angles, leg and trunk muscle activity and handle contact forces were acquired. The analysis of results show that COP excursions are significantly smaller when the subjects utilize supportive hand contact and that the displacement of COP is strongly correlated to the perturbation force and significantly larger in the anterior than posterior direction. Regression analysis of hand forces revealed that subjects utilized the hand support significantly more during the posterior than anterior perturbations. Moreover, kinematical analysis showed that utilization of supportive hand contacts alter posture of the whole body and that postural readjustments after the release of the handle, occur at different time scales in the hip, knee and ankle joints. Overall, our findings show that supportive hand contacts are efficiently used for balance control during continuous postural perturbations and that utilization of a handle has significant immediate and transitionary effects on whole body posture.},

keywords = {Ergonomy, Postural Balance, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Gams2016,

title = {Adaptation and coaching of periodic motion primitives through physical and visual interaction},

author = {Andrej Gams and Tadej Petri\v{c} and Martin Do and Bojan Nemec and Jun Morimoto and Tamim Asfour and Ale\v{s} Ude},

url = {http://linkinghub.elsevier.com/retrieve/pii/S0921889015001992},

doi = {10.1016/j.robot.2015.09.011},

issn = {09218890},

year  = {2016},

date = {2016-01-01},

journal = {Robotics and Autonomous Systems},

volume = {75},

pages = {340--351},

publisher = {Elsevier B.V.},

abstract = {In this paper we propose and evaluate a control system to (1) learn and (2) adapt robot motion for continuous non-rigid contact with the environment. We present the approach in the context of wiping surfaces with robots. Our approach is based on learning by demonstration. First an initial periodic motion, covering the essence of the wiping task, is transferred from a human to a robot. The system extracts and learns one period of motion. Once the user/demonstrator is content with the motion, the robot seeks and establishes contact with a given surface, maintaining a predefined force of contact through force feedback. The shape of the surface is encoded for the complete period of motion, but the robot can adapt to a different surface, perturbations or obstacles. The novelty stems from the fact that the feedforward component is learned and encoded in a dynamic movement primitive. By using the feedforward component, the feedback component is greatly reduced if not completely canceled. Finally, if the user is not satisfied with the periodic pattern, he/she can change parts of motion through predefined gestures or through physical contact in a manner of a tutor or a coach. The complete system thus allows not only a transfer of motion, but a transfer of motion with matching correspondences, i.e. wiping motion is constrained to maintain physical contact with the surface to be wiped. The interface for both learning and adaptation is simple and intuitive and allows for fast and reliable knowledge transfer to the robot. Simulated and real world results in the application domain of wiping a surface are presented on three different robotic platforms. Results of the three robotic platforms, namely a 7 degree-of-freedom Kuka LWR-4 robot, the ARMAR-IIIa humanoid platform and the Sarcos CB-i humanoid robot, depict different methods of adaptation to the environment and coaching.},

keywords = {Dynamic Motion, Force Control},

pubstate = {published},

tppubtype = {article}

}

@article{Denisa2016,

title = {Learning Compliant Movement Primitives Through Demonstration and Statistical Generalization},

author = {Miha Deni\v{s}a and Andrej Gams and Ale\v{s} Ude and Tadej Petri\v{c}},

url = {http://ieeexplore.ieee.org/document/7360201/},

doi = {10.1109/TMECH.2015.2510165},

issn = {1083-4435},

year  = {2016},

date = {2016-01-01},

journal = {IEEE/ASME Transactions on Mechatronics},

volume = {21},

number = {5},

pages = {2581--2594},

abstract = {In this paper, we address the problem of simultaneously achieving low trajectory tracking errors and compliant control without using explicit mathematical models of task dynamics. To achieve this goal, we propose a new movement representation called compliant movement primitives (CMPs), which encodes position trajectory and associated torque profiles and can be learned from a single user demonstration. With the proposed control framework, the robot can remain compliant and consequently safe for humans sharing its workspace, even if high trajectory tracking accuracy is required. We developed a statistical learning approach that can use a database of existing CMPs and compute new ones, adapted for novel task variations. The proposed approach was evaluated on a Kuka LWR-4 robot performing 1) a discrete pick-and-place task with objects of varying weight and 2) a periodic handle turning operation. The evaluation of the discrete task showed a 15-fold decrease of the tracking error while exhibiting compliant behavior compared to the standard feedback control approach. It also indicated no significant rise in the tracking error while using generalized primitives computed by the statistical learning method. With respect to unforeseen collisions, the proposed approach resulted in a 75% drop of contact forces compared to standard feedback control. The periodic task demonstrated on-line use of the proposed approach to accomplish a task of handle turning.},

keywords = {Compliance and Impedance Control, Machine Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2016g,

title = {Transferring knowledge during dyadic interaction: The role of the expert in the learning process},

author = {Edwin Johnatan Avila-Mireles and Dalia {De Santis} and Pietro Morasso and Jacopo Zenzeri},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=\&arnumber=7591154},

doi = {10.1109/EMBC.2016.7591154},

issn = {1557170X},

year  = {2016},

date = {2016-01-01},

journal = {Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS},

volume = {2016-Octob},

pages = {2149-2152},

abstract = {Physical interaction between man and machines is increasing the interest of the research as well as the industrial community. It is known that physical coupling between active persons can be beneficial and increase the performance of the dyad compared to an individual. However, the factors that may result in performance benefits are still poorly understood. The aim of this work is to investigate how the different initial skill levels of the interacting partners influence the learning of a stabilization task. Twelve subjects, divided in two groups, trained in couples in a joint stabilization task. In the first group the couples were composed of two naive, while in the second a naive was trained together with an expert. Results show that training with an expert results in the greatest performance in the joint task. However, this benefit is not transferred to the individual when performing the same task bimanually.},

keywords = {Human Motor Control, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2016h,

title = {Skill transfer and generalization after robot - mediated dyadic training},

author = {Edwin Johnatan Avila-Mireles and Dalia {De Santis} and Valentina Squeri and Pietro Morasso and Jacopo Zenzeri},

year  = {2016},

date = {2016-01-01},

journal = {Human Friendly Robotics},

number = {Human - Robot Interaction},

pages = {1-6},

abstract = {Several studies have investigated how the interaction between two people or between a person and a robot can be harnessed to improve the skills of the partners. Indeed, solving a task as a dyad can lead the individual to perform better than by himself. The goal of this work is to investigate how the skill level of the partner and different interactive conditions affect learning of a novel task. In particular we considered the case of partners with different initial skill level (na\"{i}ve or experts) and the influence of prior individual practice. Twenty two subjects trained in a joint stabilization task for 4 days. On the last day we tested their ability to perform the same task individually. The results show that training with a skilled partner, despite bringing to a faster learning in the joint task, does not facilitate skill transfer in the absence of individual prior practice. This suggests that the physical coupling with an expert partner may interfere with learning due to the formation of a non-veridical internal representation of the task},

keywords = {Human Motor Control, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Zorko2015a,

title = {The Waist Width of Skis Influences the Kinematics of the Knee Joint in Alpine Skiing.},

author = {Martin Zorko and Bojan Nemec and Jan Babi\v{c} and Bla\v{z} Le\v{s}nik and Matej Supej},

url = {https://www.jssm.org/hfabst.php?id=jssm-14-606.xml},

issn = {1303-2968},

year  = {2015},

date = {2015-01-01},

journal = {Journal of sports science \& medicine},

volume = {14},

number = {3},

pages = {606--19},

abstract = {Recently alpine skis with a wider waist width, which medially shifts the contact between the ski edge and the snow while turning, have appeared on the market. The aim of this study was to determine the knee joint kinematics during turning while using skis of different waist widths (65mm, 88mm, 110mm). Six highly skilled skiers performed ten turns on a predefined course (similar to a giant slalom course). The relation of femur and tibia in the sagital, frontal and coronal planes was captured by using an inertial motion capture suit, and Global Navigation Satellite System was used to determine the skiers' trajectories. With respect of the outer ski the knee joint flexion, internal rotation and abduction significantly decreased with the increase of the ski waist width for the greatest part of the ski turn. The greatest abduction with the narrow ski and the greatest external rotation (lowest internal rotation) with the wide ski are probably the reflection of two different strategies of coping the biomechanical requirements in the ski turn. These changes in knee kinematics were most probably due to an active adaptation of the skier to the changed biomechanical conditions using wider skis. The results indicated that using skis with large waist widths on hard, frozen surfaces could bring the knee joint unfavorably closer to the end of the range of motion in transversal and frontal planes as well as potentially increasing the risk of degenerative knee injuries. Key pointsThe change in the skis' waist width caused a change in the knee joint movement strategies, which had a tendency to adapt the skier to different biomechanical conditions.The use of wider skis or, in particular, skis with a large waist width, on a hard or frozen surface, could unfavourably bring the knee joint closer to the end of range of motion in transversal and frontal planes as well as may potentially increase the risk of degenerative knee injuries.The overall results of the abduction and internal rotation in respect to turn radii and ground reaction forces indicated that the knee joint movements are likely one of the key points in alpine skiing techniques. However, the skiing equipment used can still significantly influence the movement strategy.},

keywords = {Human Motor Control, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{Gorjanc2015,

title = {Koro\v{s}ka 8000: digit responses to cold stress following himalayan expedition to broadpeak, Pakistan (8051 m)},

author = {Jurij Gorjanc and Shawnda Morrison and Adam McDonnell and Jan Babi\v{c} and Igor B Mekjavi\'{c}},

url = {http://extremephysiolmed.biomedcentral.com/articles/10.1186/2046-7648-4-S1-A43},

doi = {10.1186/2046-7648-4-S1-A43},

issn = {2046-7648},

year  = {2015},

date = {2015-01-01},

journal = {Extreme Physiology \& Medicine},

volume = {4},

number = {Suppl 1},

pages = {A43},

abstract = {We investigated the effects chronic hypobaric hypoxia exposure would have on alpinists' physiological adapta- tions, including: aerobic fitness, body composition, haema- tological variables and digit perfusion responses to cold stress, performed before and immediately after a 35 day high altitude climbing expedition.},

keywords = {Sport},

pubstate = {published},

tppubtype = {article}

}

@article{DeSantis2015a,

title = {Dealing with instability in bimanual and collaborative tasks},

author = {Dalia {De Santis} and Edwin Johnatan Avila-Mireles and Valentina Squeri and Pietro Morasso and Jacopo Zenzeri},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=\&arnumber=7318635},

doi = {10.1109/EMBC.2015.7318635},

issn = {1557170X},

year  = {2015},

date = {2015-01-01},

journal = {Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS},

volume = {2015-Novem},

number = {i},

pages = {1417-1420},

publisher = {IEEE},

abstract = {In the context of unstable tasks, whenever the dynamics of the interaction are unknown, our ability to control an object depends on the predictability of the sensory feedback generated from the physical coupling at the interface with the object. In the case of physical human-human interaction, the haptic sensory feedback plays a primary role in the construction of a shared motor plan, being the channel for the mutual sharing of intentions. The present work addresses the issue of strategy selection in contexts in which instability is arising both from the environment, i.e. controlling a compliant object subject to nonlinear forces, and from the interaction with a partner, i.e. carrying out a bimanual balancing task in the presence of disturbing force-fields.},

keywords = {Human Motor Control, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2015a,

title = {Haptically Induced EMG Patterns for Upper Limb Neuromuscular Evaluation},

author = {Edwin Johnatan Avila-Mireles and Francisco J Ruiz-s\'{a}nchez},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Proceedings of 17th International Conference of the Engineering in Medicine and Biology Society},

volume = {43},

pages = {2012},

abstract = {In this paper, we present the upper-limb sEMG signals induced by a kinesthetic stimulation, provided by a haptic device in a guiding mode, with cyclic movements on the transverse plane. We show the existence of common patterns of muscular activation in healthy volunteers describing the desired sequence of signals to produce an appropriated and coordinated movement. These patterns introduce physiological information about the etiology of the movement, and a mean to improve the assessment of the neuromuscular state in a patient.},

keywords = {Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Avila-Mireles2015b,

title = {Motor control strategies in the bimanual stabilization of an unstable virtual tool},

author = {Edwin Johnatan Avila-Mireles and Dalia {De Santis} and Valentina Squeri and Pietro Morasso and Jacopo Zenzeri},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=\&arnumber=7319140},

doi = {10.1109/EMBC.2015.7319140},

issn = {1557170X},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS},

volume = {2015-Novem},

pages = {3472-3475},

abstract = {Previous works have shown that, when dealing with instabilities in a bimanual manipulation paradigm, humans modulate the stiffness of the arms according to feedforward or feedback mechanisms as a function of the dynamics of the task. The aim of this work is to complement these results getting insights on how the CNS controls the muscles to achieve the stabilization goal in the two aforementioned control strategies. Surface EMG was recorded from 13 muscles of each arm and trunk while three expert subjects performed bimanual balancing of a virtual underactuated tool immersed in an unstable force-field. Results suggest the existence of an intermittent muscle ensemble recruitment that follows two distinct activation patterns, namely synchronous co-contractions and independent activations. The observed EMG patterns were independent of the motor control strategy applied in the task. These findings therefore suggest the existence of separate control strategies for the tool stabilization and the control of hand movements at the muscular level during a bimanual postural task.},

keywords = {Human Motor Control, Physical Human Robot Interaction, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Nemec2014,

title = {Estimation of Alpine Skier Posture Using Machine Learning Techniques},

author = {Bojan Nemec and Tadej Petri\v{c} and Jan Babi\v{c} and Matej Supej},

url = {https://www.mdpi.com/1424-8220/14/10/18898},

doi = {10.3390/s141018898},

issn = {1424-8220},

year  = {2014},

date = {2014-10-01},

journal = {Sensors},

volume = {14},

number = {10},

pages = {18898--18914},

abstract = {High precision Global Navigation Satellite System (GNSS) measurements are becoming more and more popular in alpine skiing due to the relatively undemanding setup and excellent performance. However, GNSS provides only single-point measurements that are defined with the antenna placed typically behind the skier's neck. A key issue is how to estimate other more relevant parameters of the skier's body, like the center of mass (COM) and ski trajectories. Previously, these parameters were estimated by modeling the skier's body with an inverted-pendulum model that oversimplified the skier's body. In this study, we propose two machine learning methods that overcome this shortcoming and estimate COM and skis trajectories based on a more faithful approximation of the skier's body with nine degrees-of-freedom. The first method utilizes a well-established approach of artificial neural networks, while the second method is based on a state-of-the-art statistical generalization method. Both methods were evaluated using the reference measurements obtained on a typical giant slalom course and compared with the inverted-pendulum method. Our results outperform the results of commonly used inverted-pendulum methods and demonstrate the applicability of machine learning techniques in biomechanical measurements of alpine skiing.},

keywords = {Machine Learning, Postural Balance, Sport},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2014,

title = {Effects of supportive hand contact on reactive postural control during support perturbations},

author = {Jan Babi\v{c} and Tadej Petri\v{c} and Luka Peternel and Nejc \v{S}arabon},

url = {http://www.sciencedirect.com/science/article/pii/S096663621400544X},

doi = {10.1016/j.gaitpost.2014.05.012},

issn = {09666362},

year  = {2014},

date = {2014-01-01},

journal = {Gait \& Posture},

volume = {40},

number = {3},

pages = {441--446},

abstract = {There are many everyday situations in which a supportive hand contact is required for an individual to counteract various postural perturbations. By emulating situations when balance of an individual is challenged, we examined functional role of supportive hand contact at different locations where balance of an individual was perturbed by translational perturbations of the support surface. We examined the effects of handle location, perturbation direction and perturbation intensity on the postural control and the forces generated in the handle. There were significantly larger centre-of-pressure (CoP) displacements for perturbations in posterior direction than for perturbations in anterior direction. Besides, the perturbation intensity significantly affected the peak CoP displacement in both perturbation directions. However, the position of the handle had no effects on the peak CoP displacement. On the contrary, there were significant effects of perturbation direction, perturbation intensity and handle position on the maximal force in the handle. The effect of the handle position was significant for the perturbations in posterior direction where the lowest maximal forces were recorded in the handle located at the shoulder height. They were comparable to the forces in the handle at eye height and significantly lower than the forces in the handle located either lower or further away from the shoulder. In summary, our results indicate that although the location of a supportive hand contact has no effect on the peak CoP displacement of healthy individuals, it affects the forces that an individual needs to exert on the handle in order to counteract support perturbations.},

keywords = {Neuromusculoskeletal Modelling, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2014b,

title = {Teaching robots to cooperate with humans in dynamic manipulation tasks based on multi-modal human-in-the-loop approach},

author = {Luka Peternel and Tadej Petri\v{c} and Erhan Oztop and Jan Babi\v{c}},

url = {http://link.springer.com/10.1007/s10514-013-9361-0},

doi = {10.1007/s10514-013-9361-0},

issn = {0929-5593},

year  = {2014},

date = {2014-01-01},

journal = {Autonomous Robots},

volume = {36},

number = {1-2},

pages = {123--136},

abstract = {We propose an approach to efficiently teach robots how to perform dynamic manipulation tasks in cooperation with a human partner. The approach utilises human sensorimotor learning ability where the human tutor controls the robot through a multi-modal interface to make it perform the desired task. During the tutoring, the robot simultaneously learns the action policy of the tutor and through time gains full autonomy. We demonstrate our approach by an experiment where we taught a robot how to perform a wood sawing task with a human partner using a two-person cross-cut saw. The challenge of this experiment is that it requires precise coordination of the robot's motion and compliance according to the partner's actions. To transfer the sawing skill from the tutor to the robot we used Locally Weighted Regression for trajectory generalisation, and adaptive oscillators for adaptation of the robot to the partner's motion.},

keywords = {Compliance and Impedance Control, Human-in-the-Loop Control, Machine Learning, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Peternel2013b,

title = {Learning of compliant human\textendashrobot interaction using full-body haptic interface},

author = {Luka Peternel and Jan Babi\v{c}},

url = {http://www.tandfonline.com/doi/abs/10.1080/01691864.2013.808305},

doi = {10.1080/01691864.2013.808305},

issn = {0169-1864},

year  = {2013},

date = {2013-09-01},

journal = {Advanced Robotics},

volume = {27},

number = {13},

pages = {1003--1012},

publisher = {Taylor \& Francis},

abstract = {We present a novel approach where a human demonstrator can intuitively teach robot full-body skills. The aim of this approach is to exploit human sensorimotor ability to learn how to operate a humanoid robot in real time to perform tasks involving interaction with the environment. The human skill is then used to design a controller to autonomously control the robot. To provide the demonstrator with the robot?s state suitable for the full-body motion control, we developed a novel method that transforms robot?s sensory readings into feedback appropriate for the human. This method was implemented through a haptic interface that was designed to exert forces on the demonstrator?s centre of mass corresponding to the state of the robot?s centre of mass. To evaluate the feasibility of this approach, we performed an experiment where the human demonstrator taught the robot how to compliantly interact with another human. The results of the experiment showed that the proposed approach allowed the human to intuitively teach the robot how to compliantly interact with a human. We present a novel approach where a human demonstrator can intuitively teach robot full-body skills. The aim of this approach is to exploit human sensorimotor ability to learn how to operate a humanoid robot in real time to perform tasks involving interaction with the environment. The human skill is then used to design a controller to autonomously control the robot. To provide the demonstrator with the robot?s state suitable for the full-body motion control, we developed a novel method that transforms robot?s sensory readings into feedback appropriate for the human. This method was implemented through a haptic interface that was designed to exert forces on the demonstrator?s centre of mass corresponding to the state of the robot?s centre of mass. To evaluate the feasibility of this approach, we performed an experiment where the human demonstrator taught the robot how to compliantly interact with another human. The results of the experiment showed that the proposed approach allowed the human to intuitively teach the robot how to compliantly interact with a human.},

keywords = {Compliance and Impedance Control, Human-in-the-Loop Control, Physical Human Robot Interaction},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2013a,

title = {Control approaches for robotic knee exoskeleton and their effects on human motion},

author = {Tadej Petri\v{c} and Andrej Gams and Tadej Debevec and Leon \v{Z}lajpah and Jan Babi\v{c}},

url = {http://www.tandfonline.com/doi/abs/10.1080/01691864.2013.804164},

doi = {10.1080/01691864.2013.804164},

issn = {0169-1864},

year  = {2013},

date = {2013-09-01},

journal = {Advanced Robotics},

volume = {27},

number = {13},

pages = {993--1002},

publisher = {Taylor \& Francis},

abstract = {In this paper we compare three noninvasive control methods for a robotic knee exoskeleton and asses their kinematic influences on the repetitive squatting motions of able-bodied human subjects. The motion of the subjects wearing the knee exoskeleton was also compared to the motion of the subjects performing the same task without using the assistance of the knee exoskeleton. We chose the squatting motion because it approximates common movements with high metabolic cost, such as standing up from a chair and ascending or descending the stairs. Beside the two classical robotic control approaches, i.e. the position control and the gravity compensation, we propose a method that is based on a single adaptive frequency oscillator combined with an adaptive Fourier series in a feedback loop. The method can extract frequency and phase of an arbitrary periodic signal in real-time. This method is particularly appropriate for controlling novel robotic assisting devices since it does not require complex signal sensing or user calibration. The results show that the total knee torque was increased while using the exoskeleton device compared to the squatting without the assistance of the exoskeleton device. In effect, there were significant kinematic adaptations observed when the exoskleton device assisted the motion of the subjects. However, no significant kinematic differences were found between different control methods. We conclude that an assistive device can augment the abilities of the able-bodied humans in the targeted joints (i.e. the joints provided with additional mechanical power) but, on the other hand, significantly alters whole body kinematics. In this paper we compare three noninvasive control methods for a robotic knee exoskeleton and asses their kinematic influences on the repetitive squatting motions of able-bodied human subjects. The motion of the subjects wearing the knee exoskeleton was also compared to the motion of the subjects performing the same task without using the assistance of the knee exoskeleton. We chose the squatting motion because it approximates common movements with high metabolic cost, such as standing up from a chair and ascending or descending the stairs. Beside the two classical robotic control approaches, i.e. the position control and the gravity compensation, we propose a method that is based on a single adaptive frequency oscillator combined with an adaptive Fourier series in a feedback loop. The method can extract frequency and phase of an arbitrary periodic signal in real-time. This method is particularly appropriate for controlling novel robotic assisting devices since it does not require complex signal sensing or user calibration. The results show that the total knee torque was increased while using the exoskeleton device compared to the squatting without the assistance of the exoskeleton device. In effect, there were significant kinematic adaptations observed when the exoskleton device assisted the motion of the subjects. However, no significant kinematic differences were found between different control methods. We conclude that an assistive device can augment the abilities of the able-bodied humans in the targeted joints (i.e. the joints provided with additional mechanical power) but, on the other hand, significantly alters whole body kinematics.},

keywords = {Compliance and Impedance Control, Exoskeleton Design and Control, Human Performance Augmentation},

pubstate = {published},

tppubtype = {article}

}

@article{Gams2013,

title = {Effects of Robotic Knee Exoskeleton on Human Energy Expenditure},

author = {Andrej Gams and Tadej Petri\v{c} and Tadej Debevec and Jan Babi\v{c}},

url = {http://ieeexplore.ieee.org/document/6414598/},

doi = {10.1109/TBME.2013.2240682},

issn = {0018-9294},

year  = {2013},

date = {2013-06-01},

journal = {IEEE Transactions on Biomedical Engineering},

volume = {60},

number = {6},

pages = {1636--1644},

abstract = {A number of studies discuss the design and control of various exoskeleton mechanisms, yet relatively few address the effect on the energy expenditure of the user. In this paper, we discuss the effect of a performance augmenting exoskeleton on the metabolic cost of an able-bodied user/pilot during periodic squatting. We investigated whether an exoskeleton device will significantly reduce the metabolic cost and what is the influence of the chosen device control strategy. By measuring oxygen consumption, minute ventilation, heart rate, blood oxygenation, and muscle EMG during 5-min squatting series, at one squat every 2 s, we show the effects of using a prototype robotic knee exoskeleton under three different noninvasive control approaches: gravity compensation approach, position-based approach, and a novel oscillator-based approach. The latter proposes a novel control that ensures synchronization of the device and the user. Statistically significant decrease in physiological responses can be observed when using the robotic knee exoskeleton under gravity compensation and oscillator-based control. On the other hand, the effects of position-based control were not significant in all parameters although all approaches significantly reduced the energy expenditure during squatting.},

keywords = {Exoskeleton Design and Control, Human Performance Augmentation, Neuromusculoskeletal Modelling},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2013,

title = {Reflexive stability control framework for humanoid robots},

author = {Tadej Petri\v{c} and Andrej Gams and Jan Babi\v{c} and Leon \v{Z}lajpah},

url = {http://link.springer.com/10.1007/s10514-013-9329-0},

doi = {10.1007/s10514-013-9329-0},

issn = {0929-5593},

year  = {2013},

date = {2013-01-01},

journal = {Autonomous Robots},

volume = {34},

number = {4},

pages = {347--361},

abstract = {In this paper we propose a general control framework for ensuring stability of humanoid robots, determined through a normalized zero-moment-point (ZMP). The proposed method is based on the modified prioritized kinematic control, which allows smooth and continuous transition between priorities. This, as long as the selected criterion is met, allows arbitrary joint movement of a robot without any regard of the consequential movement of the ZMP. On the other hand, it constrains the movement when the criterion approaches a critical condition. The critical condition thus triggers a reflexive, subconscious behavior, which has a higher priority than the desired, conscious movement. The transition between the two is smooth and reversible. Furthermore, the switching is encapsulated in a single modified prioritized task control equation. We demonstrate the properties of the algorithm on two human-inspired robots developed in our laboratory; a human-inspired leg-robot used for imitating human movement and a skiing robot.},

keywords = {Dynamic Motion, Human-in-the-Loop Control, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2013c,

title = {Smooth continuous transition between tasks on a kinematic control level: Obstacle avoidance as a control problem},

author = {Tadej Petri\v{c} and Leon \v{Z}lajpah},

url = {http://linkinghub.elsevier.com/retrieve/pii/S0921889013000833},

doi = {10.1016/j.robot.2013.04.019},

issn = {09218890},

year  = {2013},

date = {2013-01-01},

journal = {Robotics and Autonomous Systems},

volume = {61},

number = {9},

pages = {948--959},

publisher = {Elsevier B.V.},

abstract = {Kinematically redundant robots allow simultaneous execution of several tasks with different priorities. Beside the main task, obstacle avoidance is one commonly used subtask. The ability to avoid obstacles is especially important when the robot is working in a human environment. In this paper, we propose a novel control method for kinematically redundant robots, where we focus on a smooth, continuous transition between different tasks. The method is based on a new and very simple null-space formulation. Sufficient conditions for the tasks design are given using the Lyapunov-based stability discussion. The effectiveness of the proposed control method is demonstrated by simulation and on a real robot. Pros and cons of the proposed method and the comparison with other control methods are also discussed.},

keywords = {Human-in-the-Loop Control, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2012,

title = {Analysis of musculoskeletal system responses to pertrubations during standing posture},

author = {Jan Babi\v{c} and Goran \v{S}korja},

url = {http://ev.fe.uni-lj.si/1-2-2012/Babic.pdf},

issn = {00135852},

year  = {2012},

date = {2012-01-01},

journal = {Elektrotehniski Vestnik/Electrotechnical Review},

volume = {79},

number = {1-2},

pages = {7--12},

abstract = {The purpose of the study is to examine the muscular responses of healthy adult subjects to support surface rotations in pitch and roll planes. We used an enhanced experimental approach compared to the traditionally used methods in the previous studies by other researchers where the support surface beneath the subject's feet randomly perturbed the equilibrium. Instead of a predetermined motion of the support surface that would cause the perturbation of the subject's equilibrium, we perturbed the equilibrium by a Stewart parallel platform. Mounted on the platform, a force plate was recording the motion of the centre-of-mass projection during the experiment. This setup enabled the subjects to actively control the orientation of the parallel platform by shifting the position of their centre-of-mass during the experiment. Using polar diagrams, we show the responses of four muscle groups during the stretch and proprioceptive reflex and the muscular responses to the visual stimuli. Compared to the results of the previous studies, muscular activities during the stretch and proprioceptive reflex are more equally distributed in all directions. Based on the motion of the subjects that we recorded using the motion capture system, we determined whether the muscles were stretching or extending during the responses to the perturbations. We also show the effectiveness of the muscles to compensate the perturbations in different directions.},

keywords = {Neuromusculoskeletal Modelling, Postural Balance},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2011b,

title = {Human sensorimotor learning for humanoid robot skill synthesis},

author = {Jan Babi\v{c} and Joshua Hale and Erhan Oztop},

url = {http://journals.sagepub.com/doi/10.1177/1059712311411112},

doi = {10.1177/1059712311411112},

issn = {1059-7123},

year  = {2011},

date = {2011-01-01},

journal = {Adaptive Behavior},

volume = {19},

number = {4},

pages = {250--263},

abstract = {Humans are very skilled at learning new control tasks, and in particular, the use of novel tools. In this article we propose a paradigm that utilizes this sensorimotor learning capacity to obtain robot behaviors, which would otherwise require manual programming by experts. The concept is to consider the target robot platform as a tool to be controlled intuitively by a human. The human is therefore provided with an interface designed to make the control of the robot intuitive, and learns to perform a given task using the robot. This is akin to the stage where a beginner learns to drive a car. After human learning, the skilled control of the robot is used to build an autonomous controller so that the robot can perform the task without human guidance. We demonstrate the feasibility of this proposal for humanoid robot skill synthesis by showing how a statically stable reaching skill can be obtained by means of this framework. In addition, we analyze the feedback interface component of this paradigm by examining a dynamics task, in which a human learns to use the motion of the body to control the posture of an inverted pendulum that approximates a humanoid robot, so that it stays upright.},

keywords = {Dynamic Motion, Human-in-the-Loop Control, Sensorimotor Learning},

pubstate = {published},

tppubtype = {article}

}

@article{Petric2011a,

title = {On-line frequency adaptation and movement imitation for rhythmic robotic tasks},

author = {Tadej Petri\v{c} and Andrej Gams and Auke J Ijspeert and Leon \v{Z}lajpah},

url = {http://journals.sagepub.com/doi/10.1177/0278364911421511},

doi = {10.1177/0278364911421511},

issn = {0278-3649},

year  = {2011},

date = {2011-01-01},

journal = {The International Journal of Robotics Research},

volume = {30},

number = {14},

pages = {1775--1788},

abstract = {In this paper we present a novel method to obtain the basic frequency of an unknown periodic signal with an arbitrary waveform, which can work online with no additional signal processing or logical operations. The method originates from non-linear dynamical systems for frequency extraction, which are based on adaptive frequency oscillators in a feedback loop. In previous work, we had developed a method that could extract separate frequency components by using several adaptive frequency oscillators in a loop, but that method required a logical algorithm to identify the basic frequency. The novel method presented here uses a Fourier series representation in the feedback loop combined with a single oscillator. In this way it can extract the frequency and the phase of an unknown periodic signal in real time and without any additional signal processing or preprocessing. The method determines the Fourier series coefficients and can be used for dynamic Fourier series implementation. The proposed method can be used for the control of rhythmic robotic tasks, where only the extraction of the basic frequency is crucial. For demonstration several highly non-linear and dynamic periodic robotic tasks are shown, including also a task where an electromyography (EMG) signal is used in a feedback loop.},

keywords = {Dynamic Motion, Kinematics},

pubstate = {published},

tppubtype = {article}

}

@article{Babic2009,

title = {A Biarticulated Robotic Leg for Jumping Movements: Theory and Experiments},

author = {Jan Babi\v{c} and Bokman Lim and Damir Omr\v{c}en and Jadran Lenar\v{c}i\v{c} and Frank C Park},

url = {http://mechanismsrobotics.asmedigitalcollection.asme.org/article.aspx?articleid=1484860},

doi = {10.1115/1.2963028},

issn = {19424302},

year  = {2009},

date = {2009-01-01},

journal = {Journal of Mechanisms and Robotics},

volume = {1},

number = {1},

pages = {011013},

abstract = {This paper investigates the extent to which biarticular actuation mechanisms\textemdashspring-driven redundant actuation schemes that extend over two joints, similar in function to biarticular muscles found in legged animals\textemdashimprove the performance of jumping and other fast explosive robot movements. Robust numerical optimization algorithms that take into account the complex dynamics of both the redundantly actuated system and frictional contact forces are developed. We then quantitatively evaluate the gains in vertical jumping vis-\`{a}-vis monoarticular and biarticular joint actuation schemes and examine the effects of spring stiffness and activation angle on overall jump performance. Both numerical simulations and experiments with a hardware prototype of a biarticular legged robot are reported.},

keywords = {Biarticular Muscle, Dynamic Motion, Robot Design},

pubstate = {published},

tppubtype = {article}

}