Improving finger coordination in young and elderly persons

Abstract

We studied the effects of a single practice session of a variable task with subject-specific adjustments of task difficulty (instability) on indices of multi-finger coordination in young and elderly persons. The main hypothesis was that practicing such a task would lead to contrasting changes in the amounts of two components of variance estimated across repetitive trials within the uncontrolled manifold (UCM) hypothesis: V UCM that had no effect on total force and V ORT that affected total force. In addition, we also expected to see strong transfer effects to a different task. A variable task with graded instability was designed to encourage use of variable solutions during the accurate production of total force with two fingers. The subjects practiced with the index and middle fingers pressing on individual force sensors. Overall, the older subjects showed lower indices of performance and higher indices of both V UCM and V ORT. After about 1 h of practice, both groups showed an increase in the index of involuntary force production by non-task fingers (enslaving). Both groups improved the indices of performance. The two variance indices showed opposite effects of practice: V ORT dropped with practice, while V UCM increased leading to an increase in the total amount of variance in the space of commands to fingers and in the index of force-stabilizing synergy. Performance in a simpler, non-practiced task improved, but there was no transfer of the changes in the structure of variance. Specifically, both variance components, V ORT and V UCM, dropped in the non-practiced task. The results show that the neural system responsible for synergies stabilizing important features of performance is highly adaptable to practice of tasks designed to encourage use of variable solutions. We view the results as highly promising for future use in populations with impaired coordination characterized by low synergy indices.

Figure 1

Typical performance of a representative subject from the elderly group in the “TUBE” and “SINE” tasks before practice (pre-test, A,C) and after practice (post-test, B,D) in a moderately challenging condition with G = 0.02. The gray dashed lines show the force target, the gray solid lines show the permissible error margins, and the black trace shows the actual force. After each trial, root mean square error score (RMS_ES) and time outside the permissible error margins (T_OUT) were displayed on the screen as performance indices.

Figure 2

Typical performance of a representative subject from the young group in the “TUBE” and “SINE” tasks before practice (pre-test, A,C) and after practice (post-test, B,D) in a moderately challenging condition with G = 0.02. The gray dashed lines show the force target, the gray solid lines show the permissible error margins, and the black trace shows the actual force. After each trial, root mean square error score (RMS_ES) and time outside the permissible error margins (T_OUT) were displayed on the screen as performance indices.

Figure 3

Median G values across subjects in each block for the elderly group (dashed line) and young group (solid line) over the practice time.

Figure 4

Indices of performance, RMS_ES and T_OUT, in the “TUBE” and “SINE tasks prior to (Pre) and after (Post) practice. Means with standard error bars are shown for the elderly (open bars) and young (black bars) groups.

Figure 5

Variance within the UCM (V_UCM), variance orthogonal to the UCM (V_ORT), total variance (V_TOT), and z-transformed index of synergy (ΔV_Z) are presented in panels A, B, C, and D, respectively. The variance indices were normalized by MVC² to facilitate across-subjects comparisons. The values in parentheses in panel D represent means of non-transformed ΔV values. All values are averaged across subjects for each group in the pre- and post-test. The error bars show the standard errors.