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.