Strategy of arm movement control is determined by minimization of neural effort for joint coordination

A novel biomechanical index for quality assessment of the upper-extremity movements in post-stroke patients

BACKGROUND

While motor recovery is preferred to compensatory movements for stroke patients with mild to moderate motion impairment, current movement quality assessments rarely reflect the differences between a patient's pre- and post-stroke movement patterns. Such comparison can help therapists to identify the rate of the restoration of premorbid motion patterns and prescribe the most effective treatment.

METHODS

This paper attempted to present a new biomechanical metric for the quality of upper-limb movements which uses the subject's optimal movements as a reference to evaluate his/her UL movement quality. To this end, an inverse optimal control algorithm was applied to find an estimation of the patient's premorbid motion patterns. The new biomechanical index was then calculated as a measure of similarity between the optimal and actual movement trajectories. In the next part, various simulation and clinimetric investigations were performed to evaluate the responses of the new index to variations of the movement quality as well as its test-retest reliability and concurrent validity.

RESULTS

Simulation-based analyses demonstrated that the proposed index, in contrast to the previous popular biomechanical indices, can successfully detect a wide range of abnormalities in motion signals. In addition, it showed good test-retest reliability (ICC = 0.89) and moderate correlation with clinical indices, Fugl-Meyer Assessment (r = 0.66), Action Research Arm Test (r = 0.47), and ABILHAND (r = 0.27).

CONCLUSIONS

Although the proposed index has the same degree of clinimetric properties as the previous metrics, the ability to identify the level of movement restoration and also various types and severities of motor disabilities may lead to better design and management of motor rehabilitation.

Biomechanical costs influence decisions made during ongoing actions

Accurate interaction with the environment relies on the integration of external information about the spatial layout of potential actions and knowledge of their costs and benefits. Previous studies have shown that when given a choice between voluntary reaching movements, humans tend to prefer actions with lower biomechanical costs. However, these studies primarily focused on decisions made before the onset of movement ("decide-then-act" scenarios), and it is not known to what extent their conclusions generalize to many real-life situations, in which decisions occur during ongoing actions ("decide-while-acting"). For example, one recent study found that biomechanical costs did not influence decisions to switch from a continuous manual tracking movement to a point-to-point movement, suggesting that biomechanical costs may be disregarded in decide-while-acting scenarios. To better understand this surprising result, we designed an experiment in which participants were faced with the decision between continuing to track a target moving along a straight path or changing paths to track a new target that gradually moved along a direction that deviated from the initial one. We manipulated tracking direction, angular deviation rate, and side of deviation, allowing us to compare scenarios where biomechanical costs favored either continuing or changing the path. Crucially, here the choice was always between two continuous tracking actions. Our results show that in this situation decisions clearly took biomechanical costs into account. Thus we conclude that biomechanics are not disregarded during decide-while-acting scenarios but rather that cost comparisons can only be made between similar types of actions.NEW & NOTEWORTHY In this study, we aim to shed light on how biomechanical factors influence decisions made during ongoing actions. Previous work suggested that decisions made during actions disregard biomechanical costs, in contrast to decisions made before movement. Our results challenge that proposal and suggest instead that the effect of biomechanical factors is dependent on the types of actions being compared (e.g., continuous tracking vs. point-to-point reaching). These findings contribute to our understanding of the dynamic interplay between biomechanical considerations and action choices during ongoing interactions with the environment.

From Motivation to Action: Action Cost Better Predicts Changes in Premovement Beta-Band Activity than Speed

Although premovement beta-band event-related desynchronization (β-ERD; 13-30 Hz) from sensorimotor regions is modulated by movement speed, current evidence does not support a strict monotonic association between the two. Given that β-ERD is thought to increase information encoding capacity, we tested the hypothesis that it might be related to the expected neurocomputational cost of movement, here referred to as action cost. Critically, action cost is greater both for slow and fast movements compared with a medium or "preferred" speed. Thirty-one right-handed participants performed a speed-controlled reaching task while recording their EEG. Results revealed potent modulations of beta power as a function of speed, with β-ERD being significantly greater both for movements performed at high and low speeds compared with medium speed. Interestingly, medium-speed movements were more often chosen by participants than low-speed and high-speed movements, suggesting that they were evaluated as less costly. In line with this, modeling of action cost revealed a pattern of modulation across speed conditions that strikingly resembled the one found for β-ERD. Indeed, linear mixed models showed that estimated action cost predicted variations of β-ERD significantly better than speed. This relationship with action cost was specific to beta power, as it was not found when averaging activity in the mu band (8-12 Hz) and gamma band (31-49 Hz) bands. These results demonstrate that increasing β-ERD may not merely speed up movements, but instead facilitate the preparation of high-speed and low-speed movements through the allocation of additional neural resources, thereby enabling flexible motor control.SIGNIFICANCE STATEMENT Heightened beta activity has been associated with movement slowing in Parkinson's disease, and modulations of beta activity are commonly used to decode movement parameters in brain-computer interfaces. Here we show that premovement beta activity is better explained by the neurocomputational cost of the action rather than its speed. Instead of being interpreted as a mere reflection of changes in movement speed, premovement changes in beta activity might therefore be used to infer the amount of neural resources that are allocated for motor planning.

The Strategy of Human Movement Control and Teaching Motor Skills in Norm and Pathology

The strategy used by the brain to organize human goal-directed movements is still debated. Here, I argue that without the knowledge of this strategy, teaching movement skills required in complex sports activities and for rehabilitation of motor disorders remains an art and can often result in inefficient techniques and misleading instructions. However, the leading joint hypothesis offers a solution to this problem. It suggests that the control strategy consists in rotation of a single ('leading') joint actively and using the biomechanical effect produced by the leading joint as the primary contributor to motion of the other ('trailing') joints. This "trailing joint control pattern" was found in a large variety of movement types. This pattern is simple even for seemingly complex movements, it can be easily verbalized, and it requires focusing attention during learning only on one or two movement elements at a time. The use of the trailing joint control strategy therefore allows development of better targeted techniques of motor learning and rehabilitation.

Control of paretic and non-paretic upper extremity during bimanual reaching after stroke

Most actions of daily life engage the two upper extremities (UEs) in a highly coordinated manner. While it is recognized that bimanual movements are impaired post-stroke, understanding how the paretic and non-paretic UE contributes to this impairment is important for future interventions. We investigated kinetic and kinematics at the shoulder, elbow, and wrist joints in the paretic and non-paretic UE in 8 individuals with chronic stroke and non-dominant UE in 8 healthy controls during unimanual and bimanual tasks. Kinematic analysis revealed little effect of stroke. However, kinetic analysis revealed that during unimanual movements, joint control was impaired during unimanual and bimanual movements in both UEs, although to a lesser extent in the non-paretic than paretic UE. During bimanual movements, joint control did not change in the paretic UE, and it further deteriorated in the non-paretic UE compared with the unimanual movements. Our findings suggest that a single session of bimanual task performance does not improve joint control of the paretic UE and it impairs control of the non-paretic UE, making it more like that of the paretic UE.

Identification of inverse kinematic parameters in redundant systems: Towards quantification of inter-joint coordination in the human upper extremity

Understanding and quantifying inter-joint coordination is valuable in several domains such as neurorehabilitation, robot-assisted therapy, robotic prosthetic arms, and control of supernumerary arms. Inter-joint coordination is often understood as a consistent spatiotemporal relation among kinematically redundant joints performing functional and goal-oriented movements. However, most approaches in the literature to investigate inter-joint coordination are limited to analysis of the end-point trajectory or correlation analysis of the joint rotations without considering the underlying task; e.g., creating a desirable hand movement toward a goal as in reaching motions. This work goes beyond this limitation by taking a model-based approach to quantifying inter-joint coordination. More specifically, we use the weighted pseudo-inverse of the Jacobian matrix and its associated null-space to explain the human kinematics in reaching tasks. We propose a novel algorithm to estimate such Inverse Kinematics weights from observed kinematic data. These estimated weights serve as a quantification for spatial inter-joint coordination; i.e., how costly a redundant joint is in its contribution to creating an end-effector velocity. We apply our estimation algorithm to datasets obtained from two different experiments. In the first experiment, the estimated Inverse Kinematics weights pinpoint how individuals change their Inverse Kinematics strategy when exposed to the viscous field wearing an exoskeleton. The second experiment shows how the resulting Inverse Kinematics weights can quantify a robotic prosthetic arm's contribution (or the level of assistance).

Anthropomorphic Reaching Movement Generating Method for Human-Like Upper Limb Robot

How to generate anthropomorphic reaching movement remains a challenging problem in service robots and human motor function repair/reconstruction equipment. However, there is no universally accepted computational model in the literature for reproducing the motion of the human upper limb. In response to the problem, this article presents a computational framework for generating reaching movement endowed with human motion characteristics that imitated the mechanism in the control and realization of human upper limb motions. This article first establishes the experimental paradigm of human upper limb functional movements and proposes the characterization of human upper limb movement characteristics and feature movement clustering methods in the joint space. Then, according to the specific task requirements of the upper limb, combined with the human sensorimotor model, the estimation method of the human upper limb natural postures was established. Next, a continuous task parametric model matching the characteristic motion class is established by using the Gaussian mixture regression method. The anthropomorphic motion generation method with the characteristics of the smooth trajectory and the ability of natural obstacle avoidance is proposed. Finally, the anthropomorphic motion generation method proposed in this article is verified by a human-like robot. The measurement index of the human-likeness degree of the trajectory is given. The experimental results show that for all four tested tasks, the human-likeness degrees were greater than 90.8%, and the trajectories' jerk generated by this method is very similar to the trajectories' jerk of humans, which validates the proposed method.

Moving in on human motor cortex. Characterizing the relationship between body parts with non-rigid population response fields

For cortical motor activity, the relationships between different body part representations is unknown. Through reciprocal body part relationships, functionality of cortical motor areas with respect to whole body motor control can be characterized. In the current study, we investigate the relationship between body part representations within individual neuronal populations in motor cortices, following a 7 Tesla fMRI 18-body-part motor experiment in combination with our newly developed non-rigid population Response Field (pRF) model and graph theory. The non-rigid pRF metrics reveal somatotopic structures in all included motor cortices covering frontal, parietal, medial and insular cortices and that neuronal populations in primary sensorimotor cortex respond to fewer body parts than secondary motor cortices. Reciprocal body part relationships are estimated in terms of uniqueness, clique-formation, and influence. We report unique response profiles for the knee, a clique of body parts surrounding the ring finger, and a central role for the shoulder and wrist. These results reveal associations among body parts from the perspective of the central nervous system, while being in agreement with intuitive notions of body part usage.

Emergence of Extreme Paw Accelerations During Cat Paw Shaking: Interactions of Spinal Central Pattern Generator, Hindlimb Mechanics and Muscle Length-Depended Feedback

Cat paw shaking is a spinal reflex for removing an irritating stimulus from paw by developing extremely high paw accelerations. Previous studies of paw shaking revealed a proximal-to-distal gradient of hindlimb segmental velocities/accelerations, as well as complex inter-joint coordination: passive motion-dependent interaction moments acting on distal segments are opposed by distal muscle moments. However, mechanisms of developing extreme paw accelerations during paw shaking remain unknown. We hypothesized that paw-shaking mechanics and muscle activity might correspond to a whip-like mechanism of energy generation and transfer along the hindlimb. We first demonstrated in experiments with five intact, adult, female cats that during paw shaking, energy generated by proximal muscle moments was transmitted to distal segments by joint forces. This energy transfer was mostly responsible for the segmental velocity/acceleration proximal-to-distal gradient. Distal muscle moments mostly absorbed energy of the distal segments. We then developed a neuromechanical model of hindlimb paw shaking comprised a half-center CPG, activating hip flexors and extensors, and passive viscoelastic distal muscles that produced length/velocity-depended force. Simulations reproduced whip-like mechanisms found experimentally: the proximal-to-distal velocity/acceleration gradient, energy transfer by joint forces and energy absorption by distal muscle moments, as well as atypical co-activation of ankle and hip flexors with knee extensors. Manipulating model parameters, including reversal of segmental inertia distal-to-proximal gradient, demonstrated important inertia contribution to developing the segmental velocity/acceleration proximal-to-distal gradient. We concluded that extreme paw accelerations during paw shaking result from interactions between a spinal CPG, hindlimb segmental inertia, and muscle length/velocity-depended feedback that tunes limb viscoelastic properties.

Time-varying motor control strategy for proximal-to-distal sequential energy distribution: insights from baseball pitching

The importance of a proximal-to-distal (P-D) sequential motion in baseball pitching is generally accepted; however, the mechanisms behind this sequential motion and motor control theories that explain which factor transfers mechanical energy between the trunk and arm segments are not completely understood. This study aimed to identify the energy distribution mechanisms among the segments and determine the effect of the P-D sequence on the mechanical efficiency of the throwing movement, focusing on the time-varying motor control. The throwing motions of 16 male collegiate baseball pitchers were measured by a motion capture system. An induced power analysis was used to decompose the system mechanical energy into its muscular and interactive torque-dependent components. The results showed that the P-D sequential energy flow during the movement was mainly attributed to three different joint controls of the energy generation and muscular torque- and centrifugal force-induced energy transfer. The trunk muscular torques provided the primary energy sources of the system mechanical energy, and the shoulder and elbow joints played the roles of the energy-transfer effect. The mechanical energy expenditure on the throwing hand and ball accounted for 72.7% of the total muscle work generated by the trunk and arm joints (329.2 J). In conclusion, the P-D sequence of the throwing motion is an effective way to utilize the proximal joints as the energy source and reduce muscular work production of the distal joints. This movement control assists in efficient throwing, and is consistent with the theory of the leading joint hypothesis.

Preferential Reaching and End-State Comfort: How Task Demands Influence Motor Planning

Various factors (e.g., hand preference, object properties) constrain reach-to-grasp in hemispace. With object use, end-state comfort (ESC) has been shown to supersede the preferential use of one hand at the midline. To assess how location, size, and orientation of objects (dowel, mallet, cup) influence preferred-hand use and ESC (N = 50; M_age = 20.83), three preferential reaching tasks were implemented. Object location influenced hand selection in all tasks, along with size (cups) and orientation (mallets). Object location and orientation influenced ESC, but only with dowels and mallets. When oriented away from the preferred hand in hemispace, there was a higher occurrence of non-preferred hand use to facilitate ESC. Overall, findings add to understanding of ESC and preferential reaching with varying task demands.

Laws of nature that define biological action and perception

We describe a physical approach to biological functions, with the emphasis on the motor and sensory functions. The approach assumes the existence of biology-specific laws of nature uniting salient physical variables and parameters. In contrast to movements in inanimate nature, actions are produced by changes in parameters of the corresponding laws of nature. For movements, parameters are associated with spatial referent coordinates (RCs) for the effectors. Stability of motor actions is ensured by the abundant mapping of RCs across hierarchical control levels. The sensory function is viewed as based on an interaction of efferent and afferent signals leading to an iso-perceptual manifold where percepts of salient sensory variables are stable. This approach offers novel interpretations for a variety of known neurophysiological and behavioral phenomena and makes a number of novel testable predictions. In particular, we discuss novel interpretations for the well-known phenomena of agonist-antagonist co-activation and vibration-induced illusions of both position and force. We also interpret results of several new experiments with unintentional force changes and with analysis of accuracy of perception of variables produced by elements of multi-element systems. Recently, this approach has been expanded to interpret motor disorders including spasticity and consequences of subcortical disorders (such as Parkinson's disease). We suggest that the approach can be developed for cognitive functions.

A simple joint control pattern dominates performance of unconstrained arm movements of daily living tasks

A trailing joint control pattern, during which a single joint is rotated actively and the mechanical effect of this motion is used to move the other joints, was previously observed during simplified, laboratory-based tasks. We examined whether this simple pattern also underlies control of complex, unconstrained arm movements of daily activities. Six tasks were analyzed. Using kinematic data, we estimated motion of 7 degrees of freedom (DOF) of the shoulder, elbow, and wrist, and the contribution of muscle and passive interaction and gravitational torques to net torque at each joint. Despite task variety, the hand was transported predominantly by shoulder and elbow flexion/extension, although shoulder external/internal rotation also contributed in some tasks. The other DOF were used to orient the hand in space. The trailing pattern represented by production of net torque by passive torques at the shoulder or elbow or both was observed during the biggest portion of each movement. Net torque generation by muscle torque at both joints simultaneously was mainly limited to movement initiation toward the targets and movement termination when returning to the initial position, and associated with needing to overcome gravity. The results support the interpretation of previous studies that prevalence of the trailing pattern is a feature of skillful, coordinated movements. The simplicity of the trailing pattern is promising for quantification of dyscoordination caused by motor disorders and formulation of straightforward instructions to facilitate rehabilitation and motor learning.

Haptic vs sensorimotor training in the treatment of upper limb dysfunction in multiple sclerosis: A multi-center, randomised controlled trial

BACKGROUND

In multiple sclerosis (MS) exercise improves upper limb functions, but it is unclear what training types are more effective.

OBJECTIVE

This study compares robot-assisted training based on haptic or sensorimotor exercise.

METHODS

41clinically definite MS subjects with upper limb impairment were randomised into two groups: (i) Haptic and (ii) Sensorimotor. Subjects in the Haptic performed a robot-assisted training protocol designed to counteract incoordination and weakness. The task -interaction with a virtual mass-spring system against a resistive load- requires coordination skills. Task difficulty and magnitude of resistive load were automatically adjusted to the individual impairment. Subjects in the Sensorimotor performed reaching movements under visual control; the robot generated no forces. Both groups underwent eight training sessions (40 min/session, 2 sessions/week). Treatment outcome were 9HPT and ARAT scores.

RESULTS

The average 9HPT score decreased from 74±9 s to 61±8 s for the Haptic and from 49±6 s to 44±6 s. We found a significant Treatment (p=.0453) and Time differences (p=.005), but no significant Treatment×Time interactions although we found that the absolute change was only significant in the Haptic group (p=.011). We observed no significant changes in the ARAT score. Participants tolerated treatments well with a low drop-out rate. In the subjects evaluated at after 12 week (11 subject in sensory-motor and 17 in haptic group) no retention of the effect was found.

CONCLUSIONS

Task oriented training may improve upper limb function in persons with MS especially in prevalent pyramidal impaired subjects without maintain the effects after three months.

CLINICAL TRIAL REGISTRATION NUMBER

NCT02711566 (clinicaltrial.gov).

Generalization of the resource-rationality principle to neural control of goal-directed movements

We review evidence that the resource-rationality principle generalizes to human movement control. Optimization of the use of limited neurocomputational resources is described by the inclusion of the "neurocomputational cost" of sensory information processing and decision making in the optimality criterion of movement control. A resulting tendency to decrease this cost can account for various phenomena observed during goal-directed movements.

Effect of Stroke on Joint Control during Reach-to-Grasp: A Preliminary Study

We investigated changes in control of inter-segmental dynamics underlying upper extremity dyscoordination caused by stroke. Individuals with stroke and healthy individuals performed a natural reach-to-grasp movement. Kinetic analysis revealed that both groups rotated the shoulder by muscle torque and used interaction torque to rotate the elbow. However, individuals with stroke used interaction torque less than healthy individuals, actively suppressing a substantial portion of it. This resulted in inefficient use of active control and dyscoordination of the upper extremity. The degree of interaction torque suppression and inefficiency of active control at the elbow positively correlated with stroke severity. The increased interaction torque suppression can be a strategy used by individuals with stroke to compensate for deficient feedforward control of this torque.

Effects of Reduced Effort on Mechanical Output Obtained From Maximum Vertical Jumps

The aim of this study was to evaluate the effect of reduced effort on maximum countermovement jumps. Groups of unskilled and skilled jumpers performed countermovement jumps without an arm swing at 100% and 50% effort. The results revealed markedly reduced jump height and work performed at 50% effort, although the maximum force and power output remained virtually unchanged. The observed differences were consistent across individuals with different jumping skills. A possible cause of differences in changes across the tested variables was a reduced countermovement depth associated with the 50% effort jumps. It is known to cause an increase in maximum force and power outputs, but not in jump height. Therefore, the jump height and work performed may be more closely related to our sense of effort when jumping, rather than our maximum force and power output. From a practical perspective, the present findings reiterate the importance of maximizing effort for making valid assessments of muscle mechanical capacities, as tested by maximal vertical jumps and, possibly, other maximum performance tasks.

The role of intersegmental dynamics in coordination of the forelimb joints during unperturbed and perturbed skilled locomotion

Joint coordination during locomotion and how this coordination changes in response to perturbations remains poorly understood. We investigated coordination among forelimb joints during the swing phase of skilled locomotion in the cat. While cats walked on a horizontal ladder, one of the cross-pieces moved before the cat reached it, requiring the cat to alter step size. Direction and timing of the cross-piece displacement were manipulated. We found that the paw was transported in space through body translation and shoulder and elbow rotations, whereas the wrist provided paw orientation required to step on cross-pieces. Kinetic analysis revealed a consistent joint control pattern in all conditions. Although passive interaction and gravitational torques were the main sources of shoulder and elbow motions for most of the movement time, shoulder muscle torque influenced movement of the entire limb at the end of the swing phase, accelerating the shoulder and causing interaction torque that determined elbow motion. At the wrist, muscle and passive torques predominantly compensated for each other. In all perturbed conditions, although all joints and the body slightly contributed to changes in the step length throughout the entire movement, the major adjustment was produced by the shoulder at the movement end. We conclude that joint coordination during the swing phase is produced mainly passively, by exploiting gravity and the limb's intersegmental dynamics, which may simplify the neural control of locomotion. The use of shoulder musculature at the movement end enables flexible responses to environmental disturbances. NEW & NOTEWORTHY This is the first study to investigate joint control during the swing phase of skilled, accuracy-dependent locomotion in the cat and how this control is altered to adapt to known and unexpected perturbations. We demonstrate that a pattern of joint control that exploits gravitational and interaction torques is used in all conditions and that movement modifications are produced mainly by shoulder muscle torque during the last portion of the movement.

Influence of striking technique on maximum striking velocities-experimental and statistical investigation

Forensic experts often have to assess injury and fatality risks in the context of violent blunt force trauma. Maximum striking velocities in one- and two-handed strikes with a rod-like implement can be of particular interest. Current literature lacks studies addressing this problem. The purpose of this study was therefore to measure and analyse maximum striking velocities in one-handed and two-handed strikes in female and male volunteers. We hypothesised higher striking velocities in two-handed strikes compared to one-handed strikes. Fifty volunteers performed one- and two-handed strikes from top to bottom using a steel rod of 65 cm length and 1000 g weight. A Qualisys™ Motion Analysis system registered displacements of reflecting markers fixed to the rod as well as to the volunteer's body. In one-handed strikes, the mean maximum striking velocity was 17.2 m/s in the female sample and 23.9 m/s in the male sample. Statistically not significantly different maximum striking velocities were found in two-handed strikes with mean values of 18.3 m/s in the female sample and 24.2 m/s in the male sample. Female and male volunteers also yielded similar mean maximum striking velocities in two-handed strikes comparing 'overhead' and 'overshoulder' striking techniques. In conclusion, the striking technique did not relevantly influence maximum striking velocities in our setup.

Inclusion of neural effort in cost function can explain perceptual decision suboptimality

A more general form of optimality approach applied to the entire behavioral paradigm should be used instead of abandoning the optimality approach. Adding the cost of information processing to the optimality criterion and taking into account some other recently proposed aspects of decision optimization could substantially increase the explanatory power of an optimality approach to modeling perceptual decision making.

Motor adaptations to local muscle pain during a bilateral cyclic task

The aim of this study was to determine how unilateral pain, induced in two knee extensor muscles, affects muscle coordination during a bilateral pedaling task. Fifteen participants performed a 4-min pedaling task at 130 W in two conditions (Baseline and Pain). Pain was induced by injection of hypertonic saline into the vastus medialis (VM) and vastus lateralis (VL) muscles of one leg. Force applied throughout the pedaling cycle was measured using an instrumented pedal and used to calculate pedal power. Surface electromyography (EMG) was recorded bilaterally from eight muscles to assess changes in muscle activation strategies. Compared to Baseline, during the Pain condition, EMG amplitude of muscles of the painful leg (VL and VM-the painful muscles, and RF-another quadriceps muscle with no pain) was lower during the extension phase [(mean ± SD): VL: -22.5 ± 18.9%; P < 0.001; VM: -28.8 ± 19.9%; P < 0.001, RF: -20.2 ± 13.9%; P < 0.001]. Consistent with this, pedal power applied by the painful leg was also lower during the extension phase (-16.8 ± 14.2 W, P = 0.001) during Pain compared to Baseline. This decrease was compensated for by an 11.3 ± 8.1 W increase in pedal power applied by the non-painful leg during its extension phase (P = 0.04). These results support pain adaptation theories, which suggest that when there is a clear opportunity to compensate, motor adaptations to pain occur to decrease load within the painful tissue. Although the pedaling task offered numerous possibilities for compensation, only between-leg compensations were systematically observed. This finding is discussed in relation to the mechanical and neural constraints of the pedaling task.

Feedforward compensation for novel dynamics depends on force field orientation but is similar for the left and right arms

There are well-documented differences in the way that people typically perform identical motor tasks with their dominant and the nondominant arms. According to Yadav and Sainburg's (Neuroscience 196: 153-167, 2011) hybrid-control model, this is because the two arms rely to different degrees on impedance control versus predictive control processes. Here, we assessed whether differences in limb control mechanisms influence the rate of feedforward compensation to a novel dynamic environment. Seventy-five healthy, right-handed participants, divided into four subsamples depending on the arm (left, right) and direction of the force field (ipsilateral, contralateral), reached to central targets in velocity-dependent curl force fields. We assessed the rate at which participants developed predictive compensation for the force field using intermittent error-clamp trials and assessed both kinematic errors and initial aiming angles in the field trials. Participants who were exposed to fields that pushed the limb toward ipsilateral space reduced kinematic errors more slowly, built up less predictive field compensation, and relied more on strategic reaiming than those exposed to contralateral fields. However, there were no significant differences in predictive field compensation or kinematic errors between limbs, suggesting that participants using either the left or the right arm could adapt equally well to novel dynamics. It therefore appears that the distinct preferences in control mechanisms typically observed for the dominant and nondominant arms reflect a default mode that is based on habitual functional requirements rather than an absolute limit in capacity to access the controller specialized for the opposite limb.