@article{Arditi2025,

title = {Inferring effort-safety trade off in perturbed squat-to-stand task by reward parameter estimation},

author = {Emir Arditi and Tjasa Kunavar and Negin Amirshirzad and Emre Ugur and Jan Babi\v{c} and Erhan Oztop},

doi = {10.1016/j.engappai.2024.109778},

issn = {09521976},

year  = {2025},

date = {2025-01-01},

urldate = {2025-01-01},

journal = {Engineering Applications of Artificial Intelligence},

volume = {142},

pages = {109778},

abstract = {In this study, an inverse reinforcement learning (IRL) method is developed to estimate the parameters of a reward function that is assumed to guide the movement of a biological or artificial agent. The workings of the method is shown on the problem of estimating the effort-safety trade-off of humans during perturbed squat-to-stand motions based on their Center of Mass (COM) trajectories. The proposed method involves data generation by reinforcement learning (RL) and a novel data augmentation mechanism followed by neural network training. After the training, the neural network acts as the reward parameter estimator given the Center of Mass (COM) trajectories as input. The performance of the developed method is assessed through systematic simulation experiments, where it is shown that the parameter estimation made by our method is significantly more accurate than the baseline of an optimized template-based IRL approach. In addition, as a proof of concept, a set of human movement data is analyzed with the developed method. The results revealed that most participants acquired a strategy that ensures low effort expenditure with a safety margin, producing COM trajectories slightly away from the effort-optimal.},

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

pubstate = {published},

tppubtype = {article}

}

@article{Kunavar2024,

title = {The Effects of Different Motor Teaching Strategies on Learning a Complex Motor Task},

author = {Tjasa Kunavar and Marko Jam\v{s}ek and Edwin Johnatan Avila-Mireles and Elmar Rueckert and Luka Peternel and Jan Babi\v{c}},

doi = {10.3390/s24041231},

issn = {1424-8220},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {Sensors},

volume = {24},

issue = {4},

pages = {1231},

abstract = {During the learning of a new sensorimotor task, individuals are usually provided with instructional stimuli and relevant information about the target task. The inclusion of haptic devices in the study of this kind of learning has greatly helped in the understanding of how an individual can improve or acquire new skills. However, the way in which the information and stimuli are delivered has not been extensively explored. We have designed a challenging task with nonintuitive visuomotor perturbation that allows us to apply and compare different motor strategies to study the teaching process and to avoid the interference of previous knowledge present in the na\"{i}ve subjects. Three subject groups participated in our experiment, where the learning by repetition without assistance, learning by repetition with assistance, and task Segmentation Learning techniques were performed with a haptic robot. Our results show that all the groups were able to successfully complete the task and that the subjects’ performance during training and evaluation was not affected by modifying the teaching strategy. Nevertheless, our results indicate that the presented task design is useful for the study of sensorimotor teaching and that the presented metrics are suitable for exploring the evolution of the accuracy and precision during learning.},

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

pubstate = {published},

tppubtype = {article}

}

@article{White2024,

title = {Resonance tuning of rhythmic movements is disrupted at short time scales: A centrifuge study},

author = {Olivier White and Victor Dehouck and Nicolas Boulanger and Fr\'{e}d\'{e}ric Dierick and Jan Babi\v{c} and Nandu Goswami and Fabien Buisseret},

doi = {10.1016/j.isci.2024.109618},

issn = {25890042},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {iScience},

volume = {27},

issue = {5},

pages = {109618},

abstract = {The human body exploits its neural mechanisms to optimize actions. Rhythmic movements are optimal when their frequency is close to the natural frequency of the system. In a pendulum, gravity modulates this spontaneous frequency. Participants unconsciously adjust their natural pace when cyclically moving the arm in altered gravity. However, the timescale of this adaptation is unexplored. Participants per- formed cyclic movements before, during, and after fast transitions between hypergravity levels (1g\textendash3g and 3g\textendash1g) induced by a human centrifuge. Movement periods were modulated with the average value of gravity during transitions. However, while participants increased movement pace on a cycle basis when gravity increased (1g\textendash3g), they did not decrease pace when gravity decreased (3g\textendash1g). We highlight asymmetric effects in the spontaneous adjustment of movement dynamics on short timescales, suggest- ing the involvement of cognitive factors, beyond standard dynamical models.},

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

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Human-in-the-loop optimization of wearable device parameters using an EMG-based objective function},

author = {Mar\'{i}a Alejandra D\'{i}az and Sander De Bock and Philipp Beckerle and Jan Babi\v{c} and Tom Verstraten and Kevin De Pauw},

url = {https://doi.org/10.1017/wtc.2024.9},

doi = {10.1017/wtc.2024.9},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {Wearable Technologies},

volume = {5},

pages = {e15},

abstract = {Advancements in wearable robots aim to improve user motion, motor control, and overall experience by minimizing energetic cost (EC). However, EC is challenging to measure and it is typically indirectly estimated through respiratory gas analysis. This study introduces a novel EMG-based objective function that captures individuals\' natural energetic expenditure during walking. The objective function combines information from electromyography (EMG) variables such as intensity and muscle synergies. First, we demonstrate the similarity of the proposed objective function, calculated offline, to the EC during walking. Second, we minimize and validate the EMG-based objective function using an online Bayesian optimization algorithm. The walking step frequency is chosen as the parameter to optimize in both offline and online approaches in order to simplify experiments and facilitate comparisons with related research. Compared to existing studies that use EC as the objective function, results demonstrated that the optimization of the presented objective function reduced the number of iterations and, when compared with gradient descent optimization strategies, also reduced convergence time. Moreover, the algorithm effectively converges toward an optimal step frequency near the user\'s preferred frequency, positively influencing EC reduction. The good correlation between the estimated objective function and measured EC highlights its consistency and reliability. Thus, the proposed objective function could potentially optimize lower limb exoskeleton assistance and improve user performance and human-robot interaction without the need for challenging respiratory gas measurements. Impact Statement Wearable devices are important in assisting people, such as patients or older adults, during rehabilitation and everyday activities like walking. Some exoskeletons have been able to reduce the energy cost of walking. However, they require a cumbersome device to quantify it, making it impractical to use in real-life scenarios. Thus, we need to identify a way to assess energetic cost using wearable technologies. To address this, we introduced an EMG-based objective function that captures insights into energetic cost through muscle dynamics and motor coordination. Then, we minimized the proposed objective function online by optimizing walking step frequencies. We found that the EMG-based objective function highly correlates with energetic cost during walking. We also found that our algorithm effectively identifies an optimal step frequency that reduces participants\' energetic cost. These findings will facilitate the customization of the assistance in wearable assistive devices and its application in real situations.},

keywords = {Exoskeleton Design and Control, Human Motor Control, Human Performance Augmentation, Human-in-the-Loop Control, Optimal Control},

pubstate = {published},

tppubtype = {article}

}

@article{Sorrentino2024,

title = {Comparison of joint kinematics between upright front squat exercise and horizontal squat exercise performed on a short arm human centrifugation},

author = {Riccardo G. Sorrentino and Edwin Johnatan Avila‐Mireles and Jan Babi\v{c} and Matej Supej and Igor B. Mekjavic and Adam C. McDonnell},

doi = {10.14814/phy2.16034},

issn = {2051-817X},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {Physiological Reports},

volume = {12},

issue = {13},

abstract = {This study compared the joint kinematics between the front squat (FS) conducted in the upright (natural gravity) position and in the supine position on a short arm human centrifuge (SAHC). Male participants ( \<italic\>N\</italic\> = 12) with no prior experience exercising on a centrifuge completed a FS in the upright position before (PRE) and after (POST) a FS exercise conducted on the SAHC while exposed to artificial gravity (AG). Participants completed, in randomized order, three sets of six repetitions with a load equal to body weight or 1.25 × body weight for upright squats, and 1 g and 1.25 g at the center of gravity (COG) for AG. During the terrestrial squats, the load was applied with a barbell. Knee (left/right) and hip (left/right) flexion angles were recorded with a set of inertial measurement units. AG decreased the maximum flexion angle (MAX) of knees and hips as well as the range of motion (ROM), both at 1 and 1.25 g. Minor adaptation was observed between the first and the last repetition performed in AG. AG affects the ability to FS in na\"{i}ve participants by reducing MAX, MIN and ROM of the knees and hip.},

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

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{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{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{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{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{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{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{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{\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{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{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{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{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{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{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-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}

}