Optimality versus variability: effect of fatigue in multi-finger redundant tasks

Impact of repetitive mouse clicking on forearm muscle fatigue and mouse aiming performance

Exercise induced performance fatigue has been shown to impair many aspects of fine motor function in the distal upper limb. However, most fatiguing protocols do not reflect the conditions experienced with computer use. The purpose of this study was to determine how a prolonged, low-force mouse clicking fatigue protocol impacts performance fatigue of the distal upper limb for gamers and non-gamers. Participants completed a total of 1 h of mouse clicking at 5 clicks per second. Muscle fatigue and performance were intermittently assessed. RMS amplitude increased for the forearm flexors throughout the fatigue protocol. Accuracy decreased following the first bout of clicking and returned to baseline values after 40-min. EDC and ECU displayed the greatest muscle activity while aiming, producing 11.4% and 12.9% of MVC, respectively. These findings indicate that mouse clicking may not result in performance fatigue, however, high levels of extensor activity may explain common injuries among gamers.

Uncontrolled Manifold Analysis of the Effects of Different Fatigue Locations on Kinematic Coordination During a Repetitive Upper-Limb Task

Fatigue at individual joints is known to affect interjoint coordination during repetitive multijoint tasks. However, how these coordination adjustments affect overall task stability is unknown. Twelve participants completed a repetitive pointing task at rest and after fatigue of the shoulder, elbow, and trunk. Upper-limb and trunk kinematics were collected. Uncontrolled manifold framework was applied to a kinematic model to link elemental variables to endpoint fingertip position. Mixed and one-way analysis of variances determined effects (phase and fatigue location) on variance components and synergy index, respectively. The shoulder fatigue condition had the greatest impact in causing increases in variance components and a decreased synergy index in the late phase of movement, suggesting more destabilization of the interjoint task caused by shoulder fatigue.

Hierarchical and synergistic organization of control variables during the multi-digit grasp of a free and an externally fixed object

In the referent control theory, grip force emerges by designating the referent aperture (R_a) as a threshold position inside the object. This study quantified R_a and investigated whether the synergistic control of digit referent coordinate (RC) and apparent stiffness (k) depend on the external mechanical constraints on the hand-held object. Subjects held a motorized handle capable of adjusting the grip width and performed static multi-digit prehension tasks in which the handle was free and externally fixed in different conditions. The RC and k of individual digits were reconstructed from the changes in digit normal forces and the positions as the grip width was modulated. RCs of the thumb and virtual finger were used to calculate the width and midpoint of R_a, and synergy indices quantifying the task-specific covariation in the space of the digit normal forces and {RC, k} variables were computed. We found that the k and width of the R_a were larger when holding a free handle than the fixed handle. The higher stiffness in the free condition could be a strategy to ensure grip stability. The midpoint of R_a was skewed toward the virtual finger, reflecting different magnitudes of k for the two digits. Further, the normal forces and control variables {RC, k} displayed synergistic covariation for stabilization of the total grasping force. Finally, the synergies were weaker when the handle was externally fixed, demonstrating the dependence of synergies on external constraints. These results add to the current literature by demonstrating that grasp control involves modulation of digit apparent stiffness in addition to the referent coordinate and by identifying the synergistic organization of the control variables during static grasp.

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Number of Trials Necessary to Apply Analysis Within the Framework of the Uncontrolled Manifold Hypothesis at Different Levels of Hierarchical Synergy Control

The uncontrolled manifold hypothesis is a method used to quantify motor synergies, defined as a specific central nervous system organization that maintains the task-specific stability of motor actions. The UCM allows for inter-trial variance analysis between consecutive trials. However, despite the large body of literature within this framework, there is no report on the number of movement repetitions required for reliable results. Based on the hypothetical hierarchical control of motor synergies, this study aims to determine the minimum number of trials necessary to achieve a good to excellent level of reliability. Thirteen young, healthy participants performed fifteen bilateral isometric contractions of elbow flexion when visual feedback was provided. The force and electromyography data were recorded to investigate synergies at different levels of hierarchical control. The intraclass correlation coefficient was used to determine the reliability of the variance indices. Based on the obtained results, at least twelve trials are required to analyze the inter-trial variance in both force and muscle synergies within the UCM framework.

Hand-Object Contact Force Estimation from Markerless Visual Tracking

We consider the problem of estimating realistic contact forces during manipulation, backed with ground-truth measurements, using vision alone. Interaction forces are usually measured by mounting force transducers onto the manipulated objects or the hands. Those are costly, cumbersome, and alter the objects' physical properties and their perception by the human sense of touch. Our work establishes that interaction forces can be estimated in a cost-effective, reliable, non-intrusive way using vision. This is a complex and challenging problem. Indeed, in multi-contact, a given motion can generally be caused by an infinity of possible force distributions. To alleviate the limitations of traditional models based on inverse optimization, we collect and release the first large-scale dataset on manipulation kinodynamics as 3.2 hours of synchronized force and motion measurements under 193 object-grasp configurations. We learn a mapping between high-level kinematic features based on the equations of motion and the underlying manipulation forces using recurrent neural networks (RNN). The RNN predictions are consistently refined using physics-based optimization through second-order cone programming (SOCP). We show that our method can successfully capture interaction forces compatible with both the observations and the way humans intuitively manipulate objects, using a single RGB-D camera.

Effect of Kinetic Degrees of Freedom on Multi-Finger Synergies and Task Performance during Force Production and Release Tasks

Complex structures present in a human body has relatively large degrees-of-freedom (DOFs) as compared to the requirement of a particular task. This phenomenon called motor redundancy initially deemed as a computational problem rather can be understood as having the flexibility to perform the certain task successfully. Hence, the purpose of our study was to examine the positive impact of extra DOFs (redundant DOFs) during force production tasks. For this purpose, an experimental setup was designed to simulate archery-like shooting, and purposeful organization of a redundant set of finger forces determined the stability of important performance variables as well as accurate and precise performance. DOFs were adjusted by changing the number of fingers explicitly involved in the task. The concept of motor synergy and computational framework of uncontrolled manifold (UCM) approach was used to quantify stability indices during finger force production. As a result, accuracy and precision of the task improved with an increase in DOFs. Also, the stability indices of net finger forces and moment increased with active DOFs of fingers. We concluded that the controller actively utilizes extra DOFs to increase the stability of the performance, which is associated with the improved accuracy and precision of the task.

Optimality and stability of intentional and unintentional actions: I. Origins of drifts in performance

We address the nature of unintentional changes in performance in two papers. This first paper tested a hypothesis that unintentional changes in performance variables during continuous tasks without visual feedback are due to two processes. First, there is a drift of the referent coordinate for the salient performance variable toward the actual coordinate of the effector. Second, there is a drift toward minimum of a cost function. We tested this hypothesis in four-finger isometric pressing tasks that required the accurate production of a combination of total moment and total force with natural and modified finger involvement. Subjects performed accurate force-moment production tasks under visual feedback, and then visual feedback was removed for some or all of the salient variables. Analytical inverse optimization was used to compute a cost function. Without visual feedback, both force and moment drifted slowly toward lower absolute magnitudes. Over 15 s, the force drop could reach 20% of its initial magnitude while moment drop could reach 30% of its initial magnitude. Individual finger forces could show drifts toward both higher and lower forces. The cost function estimated using the analytical inverse optimization reduced its value as a consequence of the drift. We interpret the results within the framework of hierarchical control with referent spatial coordinates for salient variables at each level of the hierarchy combined with synergic control of salient variables. The force drift is discussed as a natural relaxation process toward states with lower potential energy in the physical (physiological) system involved in the task.

Analytical Inverse Optimization in Two-Hand Prehensile Tasks

The authors explored application of analytical inverse optimization (ANIO) method to the normal finger forces in unimanual and bimanual prehensile tasks with discrete and continuously changing constraints. The subjects held an instrumented handle vertically with one or two hands. The external torque and grip force changed across trials or within a trial continuously. Principal component analysis showed similar percentages of variance accounted for by the first two principal components across tasks and conditions. Compared to unimanual tasks, bimanual tasks showed significantly more frequent inability to find a cost function leading to a stable solution. In cases of stable solutions, similar second-order polynomials were computed as cost functions across tasks and condition. The bimanual tasks, however, showed significantly worse goodness-of-fit index values. The authors show that ANIO can be used in tasks with slowly changing constraints making it an attractive tool to study optimality of performance in special populations. They also show that ANIO can fail in multifinger tasks, likely due to irreproducible behavior across trials, more likely to happen in bimanual tasks compared to unimanual tasks.

Intrinsic movement variability at work. How long is the path from motor control to design engineering?

For several years, increasing numbers of studies have highlighted the existence of movement variability. Before that, it was neglected in movement analysis and it is still almost completely ignored in workstation design. This article reviews motor control theories and factors influencing movement execution, and indicates how intrinsic movement variability is part of task completion. These background clarifications should help ergonomists and workstation designers to gain a better understanding of these concepts, which can then be used to improve design tools. We also question which techniques--kinematics, kinetics or muscular activity--and descriptors are most appropriate for describing intrinsic movement variability and for integration into design tools. By this way, simulations generated by designers for workstation design should be closer to the real movements performed by workers. This review emphasises the complexity of identifying, describing and processing intrinsic movement variability in occupational activities.

Prehension synergies during fatigue of a single digit: adaptations in control with referent configurations

The effects of muscle fatigue on the stability of precision grasps are not well known. The purpose of the current study was to investigate the effects of exercise-induced fatigue of a digit on prehension synergies in a static precision grasp. One group of participants performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). Grasp force and load-resisting force-stabilizing synergies were weaker during fatigue for group-thumb and showed no significant change for group-index. These results indicate that fatiguing the thumb compromises the stability of the precision grasp more than when the index finger is fatigued. Our results support the idea of hierarchical organization of prehension control. We proffer an explanation of our results based on two control constructs: a) Principle of superposition. This principle states that prehension can be viewed as a superposition of two independent processes controlling the slip and the tilt of the object respectively; and b) The referent configuration hypothesis. According to this hypothesis, the neural control of actions is associated with defining a set of referent values for task-related coordinates (given an external force field) defined as the referent configuration.

Effect of aging on inter-joint synergies during machine-paced assembly tasks

In recent years, uncontrolled manifold (UCM) analysis has emerged as an important method to study variability of human movements. The current study investigated the upper extremity movements during typical assembly tasks using the framework of the UCM analysis. Younger and older participants performed machine-paced assembly tasks, while the kinematics of upper extremities were recorded using a motion tracking system. The upper extremity was modeled as a 7 degrees-of-freedom system. The variance of joint angles within the UCM space (V UCM) and the variance perpendicular to the UCM space (V ORT) were analyzed. The results indicated that V UCM were not significantly different for the older and younger groups. For the older group, V ORT was significantly less than the younger group and resulted in less total variance (V TOT) and a better synergy level (Z ΔV ). Therefore, the synergies of upper extremity movement may not be impaired for machine-paced tasks as people age. While current results showed a different effect of aging on the synergies of body movement compared with one previous study, they were in line with a recently proposed theory that for natural tasks, aging people did not have impairment in the ability to organize upper extremity movement into synergies.

Contrasting effects of fatigue on multifinger coordination in young and older adults

We investigated the effects of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of the right hand on performance in MVC and accurate cyclic force production tasks in right-handed young (Young group) and strength-matched elderly (Elderly group) participants. We hypothesized that, before fatigue, the Elderly group would show weaker force-stabilizing synergies and smaller adaptive changes in the synergy index during fatigue. Synergies were defined as covaried adjustments of neural commands to fingers (finger modes) across trials that stabilize total force. Fatigue caused a significant reduction in the MVC, which was larger in the Young group compared with the Elderly group for both fatigued finger (index finger) and four fingers (index, middle, ring, and little fingers pressing together). Indexes of finger enslaving (lack of individuation) increased with fatigue in both groups. The index of force-stabilizing synergies was similar for the two groups before fatigue, while its increase with fatigue was significantly larger in the Elderly group compared with the Young group. We infer that changes in the indexes of finger interaction (enslaving) and coordination (synergy) with age seem to be correlated with changes in muscle strength. This correlation may be causally related to the progressive death of neurons at different levels of the neuromotor hierarchy. The surprisingly large changes in the synergy index with fatigue in older adults suggest that, by itself, aging does not necessarily lead to impairment in synergic control. Strength training may be a method to avoid age-related decrement in finger interaction and coordination.

Optimization and variability of motor behavior in multifinger tasks: what variables does the brain use?

The neural control of movement has been described using different sets of elemental variables. Two possible sets of elemental variables have been suggested for finger pressing tasks: the forces of individual fingers and the finger commands (also called finger modes or central commands). The authors analyzed which of the 2 sets of the elemental variables is more likely used in the optimization of the finger force sharing and which set is used for the stabilization of performance. They used two recently developed techniques-the analytical inverse optimization (ANIO) and the uncontrolled manifold (UCM) analysis-to evaluate each set of elemental variables with respect to both aspects of performance. The results of the UCM analysis favored the finger commands as the elemental variables used for performance stabilization, while ANIO worked equally well on both sets of elemental variables. A simple scheme is suggested as to how the CNS could optimize a cost function dependent on the finger forces, but for the sake of facilitation of the feed forward control it substitutes the original cost function by a cost function, which is convenient to optimize in the space of finger commands.

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

We investigated the effects of exercise-induced fatigue of a digit on the biomechanics of a static prehension task. The participants were divided into two groups. One group performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). We analyzed the prehensile action as being based on a two-level hierarchy. Our first hypothesis was that fatigue of the thumb would have stronger effects at the upper level (action shared between the thumb and all four fingers combined-virtual finger) and fatigue of the index finger would have stronger effects at the lower level of the hierarchy (action of the virtual finger shared among actual fingers). We also hypothesized that fatigue would cause a decrease in the normal force applied by the exercised digit and correspondingly lead to a decrease in the normal force applied by the opposing digit(s). Our third hypothesis was that fatigue would leave the tangential forces unaffected. Fatigue led to a significant drop in the normal force of both exercised and non-exercised (opposing) digits. The tangential forces of the exercised digits increased after fatigue. This led to a drop in the safety margin in the group-thumb, but not group-index. As such, the results supported the first two hypotheses but not the third hypothesis. Overall, the results suggested that fatigue triggered a chain reaction that involved both forces and moments of force produced by individual digits leading to a violation of the principle of superposition. The findings are interpreted within the framework of the referent configuration hypothesis.

End-state comfort and joint configuration variance during reaching

This study joined two approaches to motor control. The first approach comes from cognitive psychology and is based on the idea that goal postures and movements are chosen to satisfy task-specific constraints. The second approach comes from the principle of motor abundance and is based on the idea that control of apparently redundant systems is associated with the creation of multi-element synergies stabilizing important performance variables. The first approach has been tested by relying on psychophysical ratings of comfort. The second approach has been tested by estimating variance along different directions in the space of elemental variables such as joint postures. The two approaches were joined here. Standing subjects performed series of movements in which they brought a hand-held pointer to each of four targets oriented within a frontal plane, close to or far from the body. The subjects were asked to rate the comfort of the final postures, and the variance of their joint configurations during the steady state following pointing was quantified with respect to pointer endpoint position and pointer orientation. The subjects showed consistent patterns of comfort ratings among the targets, and all movements were characterized by multi-joint synergies stabilizing both pointer endpoint position and orientation. Contrary to what was expected, less comfortable postures had higher joint configuration variance than did more comfortable postures without major changes in the synergy indices. Multi-joint synergies stabilized the pointer position and orientation similarly across a range of comfortable/uncomfortable postures. The results are interpreted in terms conducive to the two theoretical frameworks underlying this work, one focusing on comfort ratings reflecting mean postures adopted for different targets and the other focusing on indices of joint configuration variance.

Comparison of interfinger connection matrix computation techniques

A hypothesis was proposed that the central nervous system controls force production by the fingers through hypothetical neural commands. The neural commands are scaled between values of 0 to 1, indicating no intentional force production or maximal voluntary contraction (MVC) force production, respectively. A matrix of interfinger connections transforms neural commands into finger forces. Two methods have been proposed to compute the interfinger connection matrix. The first method uses only single finger MVC trials and multiplies the interfinger connection matrix by a gain factor. The second method uses a neural network model based on experimental data. The performance of the two methods was compared on the MVC data and on a data set of submaximal forces, collected over a range of total forces and moments of force. The methods were compared in terms of (1) ability to predict finger forces, (2) accuracy of neural command reconstruction, and (3) preserved planarity of force data for submaximal force production task. Both methods did a reasonable job of predicting the total force in multifinger MVC trials; however, the neural network model performed better in regards to all other criteria. Overall, the results indicate that for modeling multifinger interaction the neural network method is preferable.

Effects of fatigue on synergies in a hierarchical system

We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of force-stabilizing synergies in both hands as well as between the hands in two-hand tasks. Synergies were defined as co-varied adjustments of commands to fingers (modes) across cycles that stabilized total force. Fatigue caused a significant reduction in the MVC of the exercised as well as the non-exercised hand. Indices of finger enslaving (lack of individuation) increased with fatigue in both hands, although the increase was significant in the exercised hand only. In contrast to the significant effects of fatigue on MVC forces performed by the non-exercised hand, there were no comparable transfer effects on the root mean square errors during accurate force production. During one-hand tasks, both hands showed high indices of force-stabilizing synergies. These indices were larger in the left hand. Fatigue led to a general increase in synergy indices. Exercise by the left hand had stronger effects on synergy indices seen in both hands. Exercise by the right hand showed ipsilateral effects only. Smaller effects of fatigue were observed on accuracy of performance of the force-down segments of the force cycles compared to the force-up segments. For the bimanual tasks, synergies were analyzed at two hierarchical levels, two-hand (four-finger) and within-a-hand (two-finger). An increase in the synergy index with fatigue was observed at the lower (two-finger) level of the hierarchy only. We interpret the lack of effects of fatigue at the upper (two-hand) level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed within the dynamic dominance hypothesis.

Forces and moments generated by the human arm: variability and control

This is an exploratory study of the accurate endpoint force vector production by the human arm in isometric conditions. We formulated three common-sense hypotheses and falsified them in the experiment. The subjects (n = 10) exerted static forces on the handle in eight directions in a horizontal plane for 25 s. The forces were of 4 magnitude levels (10, 20, 30 and 40 % of individual maximal voluntary contractions). The torsion moment on the handle (grasp moment) was not specified in the instruction. The two force components and the grasp moment were recorded, and the shoulder, elbow, and wrist joint torques were computed. The following main facts were observed: (a) While the grasp moment was not prescribed by the instruction, it was always produced. The moment magnitude and direction depended on the instructed force magnitude and direction. (b) The within-trial angular variability of the exerted force vector (angular precision) did not depend on the target force magnitude (a small negative correlation was observed). (c) Across the target force directions, the variability of the exerted force magnitude and directional variability exhibited opposite trends: In the directions where the variability of force magnitude was maximal, the directional variability was minimal and vice versa. (d) The time profiles of joint torques in the trials were always positively correlated, even for the force directions where flexion torque was produced at one joint and extension torque was produced at the other joint. (e) The correlations between the grasp moment and the wrist torque were negative across the tasks and positive within the individual trials. (f) In static serial kinematic chains, the pattern of the joint torques distribution could not be explained by an optimization cost function additive with respect to the torques. Plans for several future experiments have been suggested.