Effects of movement duration on error compensation in periodic bimanual isometric force production.
Recent studies using bimanual force production have examined how factors influence redundancy in the nervous system. The present study examined effects of different movement durations on bimanual force control strategies. Ten healthy male participants produced periodic isometric forces such that the sum of two finger forces was a target cycling between 5 and 10 % of maximum voluntary contraction during five movement durations (500, 750, 1,000, 1,250, and 1,500 ms). Correlations between the two finger forces changed from positive to negative with an increase in duration. The polynomial regression analysis indicates that while the correlations between two finger forces were most negative at the target duration of 1,250 ms, they became more positive as the durations deviated from 1,250 ms. Similarly, while force variability was smallest at the target duration of 1,250 ms, it increased as the durations deviated from 1,250 ms. These findings suggested that while the duration of 1,250 ms might be a natural frequency of both fingers, bimanual force strategies changed from force error compensation to force coupling as the durations deviated from 1,250 ms. In addition, while the variance in the sum of two finger forces (the task-relevant variance) decreased with movement duration, the difference between both the finger forces (the task-irrelevant variance) did not change with the duration. Thus, a decrease in the task-relevant variance with movement duration resulted in the negative correlation between the two finger forces and the small force variability.