MOTOR REDUNDANCY DURING MAXIMAL VOLUNTARY CONTRACTION IN 4-FINGER TASKS

The goal of the study was to investigate force-sharing patterns in mul ti-finger tasks. Maximal normal force (MNF) as well as the force-time curves produced by individual fingers were measured in 10 young male s ubjects in three tasks: (1) holding an instrumented handle in a pad op position with the thumb at seven different locations, from opposing th e index finger (L-0) to opposing the little finger (L-6); (2) holding the handle in a pad opposition with the thumb at an individually selec ted comfortable location; and (3) pressing with the four fingers again st the same handle fixed to the external support. We found that: (1) T he moment due to the normal finger forces changed systematically when the thumb position varied from L-0 to L-5/L-6, and it was equal to zer o at a certain middle position of the thumb, the neutral position. At this position, the shear force produced by the fingers was zero. (2) T he total MNF changed in an ascending-descending manner when the thumb position varied from L-0 to L-5/L-6. The highest value of the maximal total normal force was produced at a position of the thumb that was pr eferred as the most comfortable position in the grip task. (3) In the press task, the neutral line - the line with respect to which the mome nt generated by the four fingers equals zero - was at the same locatio n as the preferred thumb position in the grip tasks. (4) Larger total normal force corresponded to smaller total shear forces. (5) In grip t asks, with the thumb in a comfortable position, the force-force relati onships among fingers were approximately linear. Hence, in these thumb positions, the force-sharing pattern was established at the beginning of the trial. At the extreme positions of the thumb, irregular patter ns of the force-force relationships were observed. (6) In trials with different thumb locations, a significant correlation was found between the maximal force produced by the index and small fingers. (7) Peak f orce exerted by individual fingers in the multi-finger tasks was much smaller than the maximal force displayed by the same fingers in the si ngle-finger tasks. The peak force depended on the thumb position and v aried from 11.3% to 65.2% of the maximal force exerted by the same fin ger in the single-finger task. With the thumb in the comfortable posit ion, the relative peak force for all fingers was approximately at the same level, 50-55%. The data are in agree ment with the hypothesis tha t the total force is shared among individual fingers, minimizing the m oment with respect to the functional hand axis.