Based on previous studies, we formulated a principle of error compensation as a major principle of synergy organization during motor tasks performed by a redundant set of effecters. Within the present study, we tested the principle by an investigation of the performance of individual fingers during isometric force production when another task was performed simultaneously. Subjects were asked to press at about 30% of the maximal contraction force with three fingers (index, middle, and ring) acting in parallel. Then, they were required to perform a series of taps at 2 Hz with one of the fingers. In all the tasks, nontapping fingers changed their force production without a time delay with the changes in the force by the tapping finger. During tapping with the index and with the middle finger, both nontapping fingers showed changes in their force negatively correlated with changes in force of the tapping finger. During tapping with the ring finger, two types of behavior could be seen in different subjects with the force of the middle finger going out of phase (group 1) or in phase (group 2) with the force of the ring finger. In both cases, the force of the index finger was out of phase with the force of the ring finger. These changes, on average, induced a compensation for the expected drop in finger force during tapping, ranging in different conditions from 94% to 102%. The ratio of forces produced by the nontapping fingers did not change during the tapping in all the cases except group 2 during ring-finger tapping, when the index finger started to generate significantly higher force as compared to the middle finger. We interpret the data as results of the action of a feed-forward central mechanism leading to parallel changes in forces produced by fingers united into a structural unit.
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