Motor redundancy during maximal voluntary contraction in four-finger tasks

The goal of the study was to investigate force-sharing patterns in multi-finger tasks. Maximal normal force (MNF) as well as the force-time curves produced by individual fingers were measured in 10 young male subjects in three tasks: (1) holding an instrumented handle in a pad opposition with the thumb at seven different locations, from opposing the 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 selected comfortable location; and (3) pressing with the four fingers against the same handle fixed to the external support. We found that: (1) The 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 zero at a certain middle position of the thumb, the neutral position. At this position, the shear force produced by the fingers was zero. (2) The 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 preferred as the most comfortable position in the grip task. (3) In the press task, the neutral line - the line with respect to which the moment generated by the four fingers equals zero - was at the same location as the preferred thumb position in the grip tasks. (4) Larger total normal force corresponded to smaller total shear forces. (5) In grip tasks, with the thumb in a comfortable position, the force-force relationships 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 patterns 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 force exerted by individual fingers in the multi-finger tasks was much smaller than the maximal force displayed by the same fingers in the single-finger tasks. The peak force depended on the thumb position and varied from 11.3% to 65.2% of the maximal force exerted by the same finger in the single-finger task. With the thumb in the comfortable position, the relative peak force for all fingers was approximately at the same level, 50-55%. The data are in agree ment with the hypothesis that the total force is shared among individual fingers, minimizing the moment with respect to the functional hand axis.