Effect of kinetic degrees of freedom in multi-finger force and moment stabilizing synergies.
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The purpose of the present study was to test the principle of motor abundance as compared to motor redundance, by investigating the changes in force stabilizing and moment stabilizing synergies for pressing tasks involving different number of fingers (IM, IR, IL, MR, ML, RL, IMR, IML, IRL, MRL, IMRL; where I=Index, M=Middle, R=Ring, L=Little). Twelve healthy right-handed subjects (6 males and 6 females, 27 4.3 years) participated in the experiment. Subjects were explicitly provided a visual feedback of forces for a constant multi finger force production task. Since subjects were explicitly given a visual feedback of their performance for the force production task, strong force stabilizing synergies were expected (Hypothesis 1). Based on the principle of abundance, we hypothesized that the force stabilizing synergies would increase with the number of fingers (Hypothesis 2). Assuming that the precise moment stabilizing synergies are conditioned by everyday prehension experiences, we hypothesized that moment stabilizing synergies, if existing, would increase with the number of fingers, since all the fingers are generally used for everyday prehension and manipulation tasks, and such tasks require a precise control of moments (Hypothesis 3). Also, if both the synergies existed simultaneously, we hypothesized that those synergies would be more prominent when more fingers are involved in the task (Hypothesis 4).It was found that strong force stabilizing synergies existed for all the finger combinations, thus confirming our first hypothesis. However, these force stabilizing synergies reduced with an increase in the number of task fingers, disproving our second hypothesis. Moment destabilizing synergies were found for the two finger combinations and no moment synergies were present for the three finger combinations. However, moment stabilizing synergies existed for the four finger combinations. This confirmed our third and fourth hypothesis. We interpret the findings an evidence for the principle of abundance for stabilization of moments during pressing tasks, regardless of the fact that only the visual feedback of forces was given to the subjects.