Effects of age and gender on finger coordination in MVC and submaximal force-matching tasks

Finger interaction during accurate multi-finger force production tasks in young and elderly persons

We addressed a hypothesis that changes in indices of finger interaction during maximal force production (MVC) tasks are accompanied by changed coordination of fingers in multi-finger accurate force production tasks. To modify relative involvement of extrinsic and intrinsic hand muscles, the subjects produced force by pressing either at their distal phalanges or at their proximal phalanges. As in earlier studies, in MVC trials, the elderly subjects showed a greater force decline when pressing at the proximal phalanges as compared to pressing at the distal phalanges. Two methods were applied to analyze finger coordination during the task of four-finger force production from zero to 30% of MVC over 5 s, at the level of finger forces (performance variables) and at the level of modes (control variables). Our previous observations of higher indices of variability during the ramp task in elderly subjects have been generalized to both sites of force application. An index of finger force covariation (the difference between the variance of the total force and the sum of the variances of individual finger forces) revealed small age related differences, which did not depend on the site of the force application. In contrast, analysis of covariation of force modes within the uncontrolled manifold (UCM) hypothesis showed much better stabilization of the time profile of the total force by young subjects. The UCM hypothesis was also used to test stabilization of the pronation/supination moment during the ramp task. Young subjects showed better moment stabilization than elderly. Age related differences in both force- and moment-stabilization effects were particularly strong during force application at the proximal phalanges. We conclude that the drop in MVC is accompanied in elderly subjects with worse coordination of control signals to fingers in multi-finger tasks. The UCM analysis was more powerful as compared to analysis of force variance profiles in revealing significant differences between the groups. This general result underscores the importance of efforts to analyze motor coordination using control rather than performance variables.

The human central nervous system needs time to organize task-specific covariation of finger forces

We studied how the central nervous system (CNS) organizes outputs of effectors in a redundant motor task. During four-finger ramp force production, finger forces show positive covariations across trials at low forces, which turn into negative covariations at a critical force value (F(CR)). Subjects performed such tasks with different target amplitudes and durations of the ramp. F(CR) showed significant linear relations to the rate of force change. The slopes of the relations varied across subjects corresponding to a critical time (T(CR)) ranging from 0.13 to 0.84 s. Across subjects, T(CR) showed no relation to maximal force production; T(CR) increased with the ramp duration. We conclude that the CNS needs a certain time to organize stabilization of total force by a negative covariation among finger forces.

Uncontrolled manifold analysis of single trials during multi-finger force production by persons with and without Down syndrome

Recently, the framework of the uncontrolled manifold (UCM) hypothesis has been used to study multi-finger synergies based on analysis of motor variability across large sets of trials. We introduce a similar method of analysis, which can be applied to single trials, and hence may be more relevant to studies of atypical populations. In one experiment, results of across-trials and single-trial UCM analysis were compared for control participants who performed accurate ramp force production trials by pressing with four fingers of the hand. Both types of analysis revealed selective stabilization of total force by co-variations of individual finger forces. The stabilization was more pronounced at higher forces. When the participants purposefully varied the relative involvement of fingers during the ramp, significantly higher UCM effects were observed. However, high-pass filtering of the data at 4 Hz made these results similar to those observed in trials with natural patterns of force production. These observations allow assessment of the contribution of processes at two levels of a hypothetical hierarchical control system to the stabilization of total force. We also applied the single-trial UCM method to re-analyze previously published data from another experiment to study the motor variability in a group of persons with Down syndrome (DS) because these persons have difficulty in motor planning and timing as well as in force stabilization. Results of single-trial UCM analysis demonstrated force stabilization in these persons. The degree of force stabilization improved significantly after three days of practice. The analysis also showed that the total pronation/supination moment generated by the four fingers with respect to the midpoint was stabilized. The degree of moment stabilization did not change with practice. We conclude that the single-trial method of UCM analysis allows the analysis of hypotheses about stabilization of different performance variables by alleged multi-finger synergies in both typical individuals and individuals with DS.

Age effects on force produced by intrinsic and extrinsic hand muscles and finger interaction during MVC tasks

Finger-pressing forces are produced by activation of the intrinsic hand muscles, which are finger specific, and the extrinsic muscles that connect to multiple fingers. We tested a hypothesis of greater weakening of intrinsic hand muscles with age and quantified associated indexes of finger interaction such as enslaving (force production by unintended fingers) and force deficit (loss of finger force in multifinger tasks compared with single-finger tasks). Twelve young (23-35 yr old) and 12 elderly (70-95 yr old) men and women performed single-finger and four-finger maximal pressing tasks, in which force was applied at the proximal phalanges (PP, the intrinsic muscles are major focal force generators) and at the distal phalanges (DP, the extrinsic muscles are focal force generators). The decline in the peak force with age was greater at PP (30%) than at DP (19%). Larger indexes of finger interaction were observed at PP (enslaving = 17.2 +/- 9.4%, force deficit = 36.1 +/- 11.1%) than at DP (enslaving = 14.9 +/- 8.8%, force deficit = 27.7 +/- 10.8%) across ages and genders. We conclude that intrinsic hand muscles show disproportionate weakening with age. The greater indexes of finger interaction in PP tests with greater involvement of intrinsic hand muscles suggest that the finger interactions are predominantly of a central origin across ages and genders.

Prehension synergies: trial-to-trial variability and hierarchical organization of stable performance

We studied multi-digit synergies as relations among digit forces and points of their application across multiple repetitions of a static prehensile task. The task required holding a grasped object (14.9 N) against different external torques. Subjects (n=6) performed 25 trials for each torque condition: -1.0, -0.5, 0, +0.5 and +1.0 Nm. In spite of the variability of individual forces and points of their application, stable performance was achieved. Individual performance variables were organized into two subsets. Variables within each subset highly correlated with each other (the coefficients of correlation were close to +/- 1.0) while there was no correlation among variables from different subsets. The two subsets were associated with two components of the prehension task: grasp control (preventing an object from slipping out of the hand) and torque control (maintaining a desired object orientation).

The effects of stroke and age on finger interaction in multi-finger force production tasks

OBJECTIVE

The main purpose of this study was to investigate changes in finger interaction after stroke with strongly unilateral motor effects. Effects of age on finger interaction were also analyzed.

METHODS

Sixteen stroke subjects and 16 control subjects produced maximal voluntary contractions with different finger combinations by one hand and by two hands simultaneously. Individual finger forces were measured. In multi-finger tasks, force deficit (FD) was quantified as the difference between the peak finger forces in single-finger tasks and in multi-finger tasks, while enslaving (ENSL) was quantified as forces produced by fingers that were not required to produce force.

RESULTS

In stroke subjects, the peak forces produced by the fingers of the impaired hand (IH) were about 36% less than those produced by the unimpaired hand. Stroke resulted in higher ENSL and decreased FD in the IH, particularly when the index and middle fingers produced force together, while aging led to higher FD and no change in ENSL. Two-hand tasks were accompanied by an additional drop in the force of individual fingers, i.e. bilateral deficit (BD). No changes in BD were observed with age or after stroke.

CONCLUSIONS

We conclude that IH function in persons after stroke is accompanied not only by a general loss of finger force but also by changes in indices of multi-finger interaction. The contrast between the significantly changed indices of one-hand multi-finger interaction and unchanged BD implies that cortical neurons mediating interhemispheric inhibition are relatively spared in unilateral stroke.

SIGNIFICANCE

The study shows that stroke leads to changes not only in finger force but also in finger interaction. The conclusion on relatively spared interhemispheric projections is potentially important for therapy of hand function in stroke survivors.

Similarities and Differences in Finger Interaction across Typical and Atypical Subpopulations

The method of multidimensional scaling was applied to matrices of finger interaction (IFM) computed for individual participants for finger force production tasks. When IFMs for young controls, elderly, and persons with Down syndrome were pooled, only two dimensions described interpersonal differences; these were related to total force and to the total amount of enslaving. When IFMs for each group were analyzed separately, subpopulation-specific dimensions were found. Potentially, this analysis can be applied to discover meaningful dimensions that reflect differences in indices of finger interaction across and within subpopulations which differ in their apparent ability to use the hand. It may also be useful for tracking changes in finger interaction that occur in the process of specialized training or motor rehabilitation.

Changes in finger coordination and responses to single pulse TMS of motor cortex during practice of a multifinger force production task

We investigated the changes in finger coordination and in finger force responses to transcranial magnetic stimulation (TMS) applied over the motor cortex associated with a single practice session of an accurate ramp force production task. Subjects pressed with their index, middle and ring fingers onto three force transducers fixed to a rigid platform that was balanced on a narrow pivot under the middle finger. The task was to produce a smoothly increasing ramp of total force from 0 to 25 N over 4 s following a visual target. Subjects performed three brief series of trials without TMS (12 trials each) in the beginning, in the middle, and in the end of the experiment. The main part of the experiment involved 173 trials, and in each of them at random times in the ramp a suprathreshold TMS pulse was applied over the hand area of the contralateral motor cortex in order to evoke a twitch in the finger flexor muscles. At the end of the experiment the subjects also performed 12 constant force production trials, and TMS was unexpectedly applied in each trial. During the ramp force trials the amplitude of the response to TMS was largely independent of the force exerted at the time of stimulation, whereas in static holding trials the amplitude of the response increased with higher levels of background contraction. Over time subjects improved their overall tracking performance: the variance of the force trajectory (VarF(TOT)), as computed over sets of unperturbed trials, declined by 60% after the first 100 trials, but there was little additional improvement after the second 100 trials. Variance in the force finger space related to the total moment with respect to the pivot also showed a decline during the first half of practice and minimal further changes during the second half. In contrast, finger force variance that did not affect either total force or total moment showed no changes after the first 100 trials and a decline during the second 100 trials. This variance component quantified per finger was significantly larger than those related to the total force and total moment. The mean size of the TMS-induced phasic force increment decreased by 12% over the course of the 200 trials. The forces evoked in the index and ring fingers gradually became more equal, reducing the total moment with respect to the pivot and improving balance. We speculate that development of a relatively low twitch force with low total moment on the pivot made it easier for subjects to continue tracking after the TMS pulse. Such changes could well be correlated with the degree of corticospinal involvement in the task. The results suggest task specific, practice-related plastic changes in neural structures involved in the responses to TMS.