Proprioceptive Re-alignment drives Implicit Sensorimotor Adaptation

Multiple learning processes contribute to successful goal-directed actions in the face of changing physiological states, biomechanical constraints, and environmental contexts. Amongst these processes, implicit sensorimotor adaptation is of primary importance, ensuring that movements remain well-calibrated and accurate. A large body of work on reaching movements has emphasized how adaptation centers on an iterative process designed to minimize visual errors. The role of proprioception has been largely neglected, thought to play a passive role in which proprioception is affected by the visual error but does not directly contribute to adaptation. Here we present an alternative to this visuo-centric framework, arguing that that implicit adaptation can be understood as minimizing a proprioceptive error, the distance between the perceived hand position and its intended goal. We use this proprioceptive re-alignment model (PReMo) to re-examine many phenomena that have previously been interpreted in terms of learning from visual errors, as well as offer novel accounts for unexplained phenomena. We discuss potential challenges for this new perspective on implicit adaptation and outline a set of predictions for future experimentation.

Computational Approaches to Music Motor Performance: Clustering of Percussion Kinematics Underlying Performance Style

The present study investigated motor kinematics underlying performance-related movements in marimba performance. Participants played a marimba while motion capture equipment tracked movements of the torso, shoulders, elbows, wrists, and hands. Principal components analysis was applied to assess the movements during the performance related to sound production and sound preparation. Subsequent cluster analyses sought to identify coupling of limb segment movements that may best characterize performance styles present in the performance. The analysis revealed four clusters that were thought to reflect performance styles of expressive performance, postural sway, energy efficiency, and a blend of the former styles. More specifically, the expressive cluster was best characterized by limb movements occurring along the vertical z-axis, whereas the postural sway cluster was characterized by forwards and backwards motions of the torso and upper limbs. The energy efficient cluster was characterized by movements of the body moving left to right along the marimba, whereas the blended style demonstrated limited delineation from the alternate styles. Such findings were interpreted as evidence that performance styles occur within a framework of biomechanical constraints and hierarchical stylistic factors. Overall, the results provided a more holistic understanding of motor execution in percussion performance.

Bimanual Grasping Adheres to Weber's Law

Weber's law states that our ability to detect changes in stimulus attributes decreases linearly with their magnitude. This principle holds true for many attributes across sensory modalities but appears to be violated in grasping. One explanation for the failure to observe Weber's law in grasping is that its effect is masked by biomechanical constraints of the hand. We tested this hypothesis using a bimanual task that eliminates biomechanical constraints. Participants either grasped differently sized boxes that were comfortably within their arm span (action task) or estimated their width (perceptual task). Within each task, there were two conditions: One where the hands’ start positions remained fixed for all object sizes (meaning the distance between the initial and final hand-positions varied with object size), and one in which the hands’ start positions adapted with object size (such that the distance between the initial and final hand-position remained constant). We observed adherence to Weber's law in bimanual estimation and grasping across both conditions. Our results conflict with a previous study that reported the absence of Weber's law in bimanual grasping. We discuss potential explanations for these divergent findings and encourage further research on whether Weber's law persists when biomechanical constraints are reduced.

Relative skull size as one of the factors limiting skull shape variation in passerines

Despite a considerable interest of researchers to the issue of variation in skull shapes of birds and factors influencing it, some drivers associated with the design features of an entire bird body, which are important for both successful terrestrial locomotion and flight, are overlooked. One of such factors, in our opinion, is relative skull size (skull length in relation to body mass), which can affect the position of the body's center of gravity. We tested effects of relative skull size, allometry (i.e. absolute skull size), and diet on variation in skull shape. The study was conducted on 50 songbird species with a wide range of body mass (8.3g to 570g) and dietary preferences (granivores, insectivores/granivores, insectivores, omnivores). Skull shape was analyzed using 2D geometric morphometrics. We revealed that similar patterns of skull shape occur among passerines with different body sizes and diets. The relative skull size predicted skull shape to a similar extent and with a similar pattern as the absolute size. In our opinion, the effect of the relative skull size on skull shape variation is likely due to biomechanical constraints related to flight.

Alterations in intermuscular coordination underlying isokinetic exercise after a stroke and their implications on neurorehabilitation

Background Abnormal intermuscular coordination limits the motor capability of stroke-affected upper limbs. By evaluating the intermuscular coordination in the affected limb under various biomechanical task constraints, the impact of a stroke on motor control can be analyzed and intermuscular coordination-based rehabilitation strategies can be developed. In this study, we investigated upper limb intermuscular coordination after a stroke during isokinetic movements. Methods Sixteen chronic stroke survivors and eight neurologically intact individuals were recruited. End-point forces and electromyographic activities of the shoulder and elbow muscles were measured while the participants performed isokinetic upper limb movements in a three-dimensional space. Intermuscular coordination of the stroke survivors and the control participants was quantified in the form of muscle synergies. Then, we compared the number, composition, and activation coefficients of muscle synergies and the end-point force between the groups. The correlation between the alteration of muscle synergies and the level of motor impairment was investigated. Results Four and five muscle synergies in the stroke and control groups were observed, respectively. The composition of muscle synergies was comparable between the groups, except that the three heads of the deltoid muscle were co-activated and formed one synergy in the stroke group, whereas those muscles formed two synergies in the control group. When the number of muscle synergies between the groups matched, the comparable composition of muscle synergies was observed in both groups. Alternatively, the modulation of synergy activation coefficients was altered after a stroke. The severity of motor impairments was negatively correlated with the similarity of the post-stroke synergies with respect to the mean control synergies. Conclusions Stroke-affected upper limbs seemed to modularize the activation of the shoulder and elbow muscles in a fairly similar way to that of neurologically intact individuals during isokinetic movements. Compared with free (i.e., unconstrained) movement, exercise under biomechanical constraints including the isokinetic constraint might promote the activation of muscle synergies independently in stroke survivors. We postulated the effect of biomechanical constraints on the intermuscular coordination and suggested a possible intermuscular coordination-based rehabilitation protocol that provides the biomechanical constraint appropriate to a trainee throughout the progress of rehabilitation.

3D Modelling Of Customized Lasts Based On Anthropometric Data Acquired From 3D Foot Scanning - One Study Case

Designing and manufacturing personalized lasts are the first steps in obtaining the right fitted footwear for various users, especially for sport or/and medical purposes. The accurate dimensional relationship between foot and last represents the key element for this activity. The critical shape of the last should always be determined by the shape of the foot and the cumulative relationship between lengths, widths, heights and girths, whatever method is used, Some corrections and constraints must always be considered because the shoe-last is not identical to the foot. The foot anthropometric measurements are modified based on biomechanical constraints and technological limitations and they are interactively transformed into last’s dimensions by using 3D modelling. The present study brings together the modern scanning technique with the new methodology for modifying a reference last, and it is aimed to explore the philosophy of re-designing functional lasts. It also tests and highlights the limits of the actual methodology for shoe-last virtual prototyping based on anthropometric data acquired from one commercially available 3D foot scanning system.

A Neurophysiological Pattern as a Precursor of Work-Related Musculoskeletal Disorders Using EEG Combined with EMG

We aimed to determine the neurophysiological pattern that is associated with the development of musculoskeletal pain that is induced by biomechanical constraints. Twelve (12) young healthy volunteers (two females) performed two experimental realistic manual tasks for 30 min each: (1) with the high risk of musculoskeletal pain development and (2) with low risk for pain development. During the tasks, synchronized electroencephalographic (EEG) and electromyography (EMG) signals data were collected, as well as pain scores. Subsequently, two main variables were computed from neurophysiological signals: (1) cortical inhibition as Task-Related Power Increase (TRPI) in beta EEG frequency band (β.TRPI) and (2) muscle variability as Coefficient of Variation (CoV) from EMG signals. A strong effect size was observed for pain measurement under the high risk condition during the last 5 min of the task execution; with muscle fatigue, because the CoV has decreased below 18%. An increase in cortical inhibition (β.TRPI >50%) was observed after the 5th min of the task in both experimental conditions. These results suggest the following neurophysiological pattern—β.TRPI ≥ 50% and CoV ≤ 18%—as a possible indicator to monitor the development of musculoskeletal pain in the shoulder in the context of repeated and prolonged exposure to manual tasks.

A Concept Design of an Adaptive Tendon Driven Mechanism for Active Soft Hand Orthosis

The Hands exert a vital role in the simplest to most complex daily tasks. Losing the ability to make hand movements, which is usually caused by spinal cord injury or stroke, dramatically impacts the quality of life. In order to counteract this problem, several assisting devices have been proposed, but they still present several usage limitations. The marketable orthoses are generally either the static type or over-expensive active orthosis that cannot perform the same degrees of freedom (DoF) that a hand can do. This paper presents a conceptual design of a tendon-driven mechanism for hand’s active orthosis. This study is a part of an effort to develop an effective and low-cost hand’s orthosis for people with hand paralysis. The tendon design proposed was thought to comply with some requisitions such as lightness and low volume, as well as fit with the biomechanical constraints of the hand joints to enable a comfortable use. The mechanism employs small cursors on the phalanges to allow the tendons to run on the dorsal side and by both sides of the fingers, allowing 2 DoF for each finger, and one extra tendon enlarges the hands’ adduction nuances. With this configuration, it is simple enough to execute the flexion and extension movements, which are the most used movements in daily actives, using one single DC actuator for one DoF to reduce manufacturing costs, or with more DC actuators to enable more natural hand coordination. This system of actuation is suitable to create soft exoskeletons for hands easily embedded into 3D printed parts, which could be merged over statics thermoplastic orthosis. The final orthosis design allows dexterous finger movements and force to grasp objects and perform tasks comfortably.

Alterations in intermuscular coordination underlying isokinetic exercise after a stroke and their implications on neurorehabilitation

Background: Abnormal intermuscular coordination limits the motor capability of stroke-affected upper limbs. By evaluating the intermuscular coordination in the affected limb under various biomechanical task constraints, the impact of a stroke on motor control can be analyzed and intermuscular coordination-based rehabilitation strategies can be developed. In this study, we investigated upper limb intermuscular coordination after a stroke during isokinetic movements. Methods: Sixteen chronic stroke survivors and eight neurologically intact individuals were recruited. End-point forces and electromyographic activities of the shoulder and elbow muscles were measured while the participants performed isokinetic upper limb movements in a three-dimensional space. Intermuscular coordination of the stroke survivors and the control participants was quantified in the form of muscle synergies. Then, we compared the number, composition, and activation coefficients of muscle synergies and the end-point force between the groups. The correlation between the alteration of muscle synergies and the level of motor impairment was investigated. Results: Four and five muscle synergies in the stroke and control groups were observed, respectively. The composition of muscle synergies was comparable between the groups, except that the three heads of the deltoid muscle were co-activated and formed one synergy in the stroke group, whereas those muscles formed two synergies in the control group. When the number of muscle synergies between the groups matched, the comparable composition of muscle synergies was observed in both groups. Alternatively, the modulation of synergy activation coefficients was altered after a stroke. The severity of motor impairments was negatively correlated with the similarity of the post-stroke synergies with respect to the mean control synergies. Conclusions: Stroke-affected upper limbs seemed to modularize the activation of the shoulder and elbow muscles in a fairly similar way to that of neurologically intact individuals during isokinetic movements. Compared with free (i.e., unconstrained) movement, exercise under biomechanical constraints including the isokinetic constraint might promote the activation of muscle synergies independently in stroke survivors. We postulated the effect of biomechanical constraints on the intermuscular coordination and suggested a possible intermuscular coordination-based rehabilitation protocol that provides the biomechanical constraint appropriate to a trainee throughout the progress of rehabilitation.