FORCE SHARING AMONG FINGERS AS A MODEL OF THE REDUNDANCY PROBLEM

The aim of this study was to test Bernstein's idea that motor synergie s provide solutions to the motor redundancy problem. Forces produced b y individual fingers of one hand were recorded in one-, two-, three-, and four-finger tasks. The subjects (n=10) were asked to produce maxim al total force (maximal voluntary contraction, MVC) and to match a ram p total force profile using different combinations of fingers. We foun d that individual finger forces were smaller in multifinger MVC tasks than in single-finger tasks. The deficit increased with the number of fingers involved. A saturation effect was observed: when several effec ters were involved, adding a new effector did not significantly change the total force output. The data confirmed the idea that the central neural drive arriving at the level of synergies has a certain limit, a ceiling, that cannot be exceeded. The central nervous system cannot m aximally activate the muscles serving all the fingers at the same time . Secondly, during the course of ramp trials, forces produced by indiv idual fingers were linearly related to each other. Hence, a force shar ing pattern was established at the beginning of the trial and did not change during the ramp period. A hypothesis is suggested that force di stribution among fingers may be organized so as to minimize unnecessar y rotational moment with respect to the functional longitudinal axis o f the hand. Finally, in the four-finger trials, variance of the total maximal force output in ten consecutive attempts was smaller than the sum of variances of the maximal individual finger forces. The finding suggests that the control system of the motor tasks studied involves a t least two levels, a central neural drive level and a synergy level. At the synergy level, an intercompensation in individual finger force production is observed.