Frédéric Danion

Structure of motor variability in marginally redundant multifinger force production tasks.

Abstract. The framework of the uncontrolled manifold hypothesis (UCM hypothesis) was applied to the analysis of the structure of finger force variability during oscillatory force production tasks. Subjects produced cycles of force with one, two (index and middle), or three (index, middle, and ring) fingers acting in parallel against force sensors mounted inside a small frame. The frame could be placed on the top of a table (stable conditions) or on a 4-mm-wide supporting surface (unstable conditions). Subjects were less variable when they used two fingers than when using one finger; adding the third finger did not change indices of variability of the performance. Components of finger force variance that did (VUN) or did not (VCOMP) change the value of a particular functional variable were computed for two control hypotheses: (1) at each time, the subjects tried to stabilize the total value of force (force-control); and (2), at each time, the subjects tried to stabilize the total moment produced with respect to an axis parallel to the hand/forearm (moment-control). Most subjects showed selective stabilization of moment and destabilization of force throughout most of the force cycle, in both stable and unstable conditions. The shapes of VUN and VCOMP suggested a possibility of selective compensation of timing errors across fingers within force cycles. One subject showed different relations between VUN and VCOMP, suggesting that these relations did in fact reflect particular central strategies of solving the tasks. The UCM method is applicable to force production tasks. It allows the comparison of control hypotheses in a quantitative way and unveils central strategies of control of redundant motor systems. Within this approach, redundancy (rather, abundance) is not a problem but an inherent part of a solution for natural motor tasks.

Understanding finger coordination through analysis of the structure of force variability

Abstract. Most common motor acts involve highly redundant effector systems. Understanding how such systems are controlled by the nervous system is a long-standing scientific challenge. Most proposals for solving this problem are based on the assumption that a particular solution, which optimizes additional constraints, is selected by the nervous system out of the many possible solutions. This study attempts to address this question in the context of coordinating individual finger forces to produce a controlled total force oscillation between 5% and 35% of each subjects maximum force of voluntary contraction, under two different combinations of four fingers. The structure of variability of individual finger forces was evaluated with respect to hypotheses that, at each instance in time, subjects attempt to: (1) stabilize the value of total force and (2) stabilize the total moment created by the fingers about the long axis passing through the forearm and midline of the hand. The results provide evidence that a range of goal-equivalent finger force combinations is generated to stabilize the values of total force and the total moment. The control of total force was specified explicitly by the task. However, it was stabilized only near the time of peak force. In contrast, the total moment was stabilized throughout most of the force cycle. The results lead to the suggestion that successful task performance is achieved, not by selecting a single optimal solution, but by discovering an appropriate control law that selectively stabilizes certain combinations of degrees of freedom relevant to the task while releasing from control other combinations.

The effect of expertise in gymnastics on postural control

The goal of this paper was (1) to investigate if gymnasts have a more stable standing posture than experts in other sports, and (2) to determine how much gymnasts are affected by the removal of vision in different postural tasks. Six expert gymnasts and six experts in other non-gymnastic sports were asked to maintain balance in three standing postures of increasing difficulty: bipedal, unipedal, and unipedal + unstable support (i.e. 7 cm thick foam surface). Each posture was tested successively with and without vision. Based on the displacement of the center of pressure (range and mean average speed), the results showed that when visual cues were available, postural sway increased with the difficulty of the task, but both groups had comparable performance in all the tasks. When vision was removed, although both groups demonstrated larger postural sway in the unipedal tasks, this effect was less accentuated for the gymnasts. We concluded that gymnasts are able to use the remaining sensory modalities to compensate for the lack of vision in unstable postures.

Stride variability in human gait: the effect of stride frequency and stride length

This study focused on spatial and temporal variability of the stride in human gait. We determined the role of stride frequency (F) and stride length (L) on those parameters. Eight healthy subjects walked on a treadmill using 25 different FL combinations (0.95<L<1.5 m, and 0.8<F<1.26 Hz). The results showed that spatial and temporal variabilities tend to increase in concert with respect to change in stride parameters. In addition, stride variability was found (1) to be minimal at F=1 Hz; and (2) to increase with smaller L. During additional trials, subjects walked freely at various speeds. Although it is generally hypothesized that freely chosen behaviors are optimal in terms of variability, our data show that this is not always the case in human gait.

A mode hypothesis for finger interaction during multi-finger force-production tasks

Abstract. Finger forces are known to change involuntarily during multi-finger force-production tasks, even when a fingers involvement in a task is not consciously changed (the enslaving effect). Furthermore, during maximal force-production (MVC) tests, the force produced by a given finger in a multi-finger task is smaller than the force generated by this finger in its single-finger MVC test (the force-deficit effect). A set of hypothetical control variables – modes – is introduced. Modes can be estimated based on individual finger forces during single-finger MVC tests. We show that a simple formal model based on modes with only one free parameter accounts for finger forces during a variety of multi-finger MVC tests. The free parameter accounts for the force-deficit effect, and its value depends only on the number of explicitly involved fingers. This approach offers a simple framework for the analysis of finger interaction during multi-finger actions.

Finger coordination during discrete and oscillatory force production tasks

We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze the structure of finger force variability in discrete (ramp) and oscillatory force production tasks performed by the index and middle fingers of the right hand acting in parallel. Subjects performed the tasks at fast and slow rates, with and without a visual template presented on the screen. The variance of finger forces was partitioned into two components, compensated variance (VCOMP), which did not affect total force, and uncompensated variance (VUN), which affected total force. Only minor effects of task (discrete or oscillatory) and of template (with or without) were seen on the variance profiles, leading us to conclude that the basic principles of synergy organization are common across discrete and oscillatory tasks. In contrast, the rate of force production had major effects on the structure of force variance. A modification of Goodman’s model of motor variability was used to analyze the dependences VUN and VCOMP on the magnitude of force and on the rate of force production. VUN showed a strong relation to the rate of force production and only weak dependence on the magnitude of force. In contrast, VCOMP showed minimal effects of the rate of force production and strong effects of the force magnitude. The findings are interpreted as demonstrations of a limitation in the ability of the central nervous system to organize a twofinger synergy such that errors in the timing of individual finger force profiles are canceling each other’s effects on the total force. In contrast, the synergy is efficiently intercompensating errors related to imprecise setting of force magnitudes of the two fingers.

Postural sway under muscle vibration and muscle fatigue in humans

Separate studies have demonstrated that vibration and fatigue of ankle muscles alter postural control. The purpose of the present experiment was to investigate the effect of ankle muscle vibration on the regulation of postural sway in bipedal stance following ankle muscle fatigue. Center of foot pressure displacements were recorded using a force platform. Results showed a similar increase in postural sway under muscle fatigue as well as under muscle vibration. Interestingly, under muscle fatigue muscle vibration did not induce a further increase in postural sway. Two hypotheses could, at least, account for this observation: (1). fatigued muscles are less sensitive to muscle vibration and (2). the central nervous system relies less upon proprioceptive information originating from fatigued muscles for regulating postural sway.

The effect of fatigue on multifinger co‐ordination in force production tasks in humans

1 This study investigated the effects of fatigue, induced by production of maximal isometric force for 60 s with four fingers, upon indices of multifinger co‐ordination. 2 Measurements of individual finger forces were performed during single‐ and multifinger maximal force production (maximal voluntary contraction, MVC) for two sites of force application, the middle of the distal or the middle of the proximal phalanxes. Two fatiguing exercises were used, involving force production at the distal phalanxes and at the proximal phalanxes. Fourteen subjects were tested. 3 The total force in four‐finger tasks dropped by about 43 % when it was produced at the site involved in the fatiguing exercise. During force production at the other site, MVC dropped by 23 %. During single‐finger MVC tests, force drop with fatigue was similar across all four fingers (about −25 % of their corresponding MVCs). 4 Force production by one finger was accompanied by involuntary force production by other fingers (enslaving). Enslaving remained unchanged by fatigue when measured during force generation at the site involved in the fatiguing exercise, but increased during force production at the other site. 5 The total MVC of four fingers acting in parallel was smaller than the sum of the MVCs of these fingers in single‐finger tasks (force deficit). The force deficit increased with fatigue. Force‐sharing patterns during four‐finger tasks showed only minor changes under fatigue. 6 These results indicate that the effects of fatigue were not limited to changes in the force‐generating capabilities of the muscles. In particular, fatigue could lead to a reorganisation at a neural level that defines commands to individual fingers.

Approaches to analysis of handwriting as a task of coordinating a redundant motor system.

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.

Central mechanisms of finger interaction during one- and two-hand force production at distal and proximal phalanges

In this study we used changes in the relative involvement of different muscle groups during force production at the distal (DT) and proximal (PR) phalanges to test and modify a hypothesis on the central organization of multi-finger control for tasks involving non-homologous elements in the two hands. Ten subjects produced maximal force with different finger combinations. Two symmetrical (PR/PR and DT/DT) and two asymmetrical (PR/DT and DT/PR) combinations of force application sites in the two hands were used. During one-hand tasks, higher forces were produced at the PR site. In multi-finger tasks, total peak force was smaller than the sum of peak forces in single-finger tasks by the involved fingers (force deficit). Force production by some fingers of a hand was accompanied by involuntary force production by other fingers (enslaving). Force deficit and enslaving were both higher at the PR site. Two-hand tasks were accompanied by an additional drop in the force of individual fingers, i.e., bilateral deficit (BD). When symmetrical sites of force production were used in the two hands, BD was lower for symmetrical finger groups than for asymmetrical groups. During tests at asymmetrical sites, BD was higher and did not depend on symmetry of involved finger groups. We conclude that within-a-hand force deficit and enslaving are likely to be of a central, neural origin. An earlier introduced hypothesis has been expanded assuming that excitatory projections to contralateral finger representations exist only for homologous elements (sub-synergies) of a multi-finger force production synergy, while only inhibitory projections connect non-homologous elements.