Force sharing among fingers as a model of the redundancy problem

Abstract

The aim of this study was to test Bernstein's idea that motor synergies provide solutions to the motor redundancy problem. Forces produced by individual fingers of one hand were recorded in one-, two-, three-, and four-finger tasks. The subjects (n=10) were asked to produce maximal total force (maximal voluntary contraction, MVC) and to match a ramp total force profile using different combinations of fingers. We found 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 effectors 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 maximally activate the muscles serving all the fingers at the same time. Secondly, during the course of ramp trials, forces produced by individual fingers were linearly related to each other. Hence, a force sharing pattern was established at the beginning of the trial and did not change during the ramp period. A hypothesis is suggested that force distribution among fingers may be organized so as to minimize unnecessary rotational moment with respect to the functional longitudinal axis of 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 at 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.

Control of finger force direction in the flexion-extension plane

“…In the four-finger MVC pressing tasks it was found that the variance of the total maximal force output was smaller than the sum of variances of the maximal individual finger forces (Li et al 1998a(Li et al , 1998b. The reduction of the force variance at the multi-finger level suggested the existence of an inter-compensation among individual digits also confirmed in a series of further studies (Latash et al 2001(Latash et al , 2002a(Latash et al , 2002b.…”

“…At the different target directions, the torques at the individual joints are different. The phenomenon of enslaving was described in the literature for the fingertip forces (Li et al 1998a(Li et al ,1998b. The dependence of the enslaved force direction on the direction of the master finger force suggests that enslaving is joint-torque-specific.…”

“…The existence of synergies in multi-finger force production is well documented and revealed in both pressing and grasping tasks (Li et al 1998a(Li et al , 1998bLatash et al 2001Latash et al , 2002aLatash et al , 2002b. In the four-finger MVC pressing tasks it was found that the variance of the total maximal force output was smaller than the sum of variances of the maximal individual finger forces (Li et al 1998a(Li et al , 1998b.…”

“…to explore the dependence of the direction of forces produced by fingers that are not required to produce force (enslaved forces; Li et al 1998aLi et al , 1998bZatsiorsky et al 1998Zatsiorsky et al , 2000 on the target direction; and 4. to establish whether there is a multi-finger synergy stabilizing the direction of the total force.…”

“…In the motor control literature, to test whether the individual variables compensate for each other, the sum of the variances of the elementary variables is commonly compared with the variance of the sum (Li et al 1998b;Latash et al 2002b). According to the Bienaimé Equality theorem for the mutually independent and integrable random variables; the sum of the variances is equal to the variance of the sum (Loevè 1955, p. 234).…”