The targeted force steps developed by a human wrist: Cyclic components in the motor program

Abstract

The targeted flexor isometric force steps developed by a wrist and corresponding EMG activity of the flexor muscles were studied in healthy volunteers. In 82.3–97.5% of trials, the main component of the force trajectory (before the postcorrections of the force level) showed complex dynamics: several (from two to five)dF/dt peaks were observed at this section of the trajectory. The distribution of time intervals from the initiation of the force trajectory todF/dt peaks was polymodal in all cases. The mean interval between successivedF/dt peaks varied from 56.7 msec to 84.4 msec in different individuals (the average group value was 70.5 msec). All tested subjects exhibited amplitude modulation of integrated EMG at the section reflecting the force step development; the averaged interval between the successive first-order EMG peaks calculated for the group of five persons was 78.6 msec. Strict temporal correlation ofdF/dt peaks and EMG peaks was found (coefficient of correlation averaged 0.94). It is concluded that the motor command ensuring performance of isometric force step includes a cyclic component; this feature must be taken into account when mechanisms controlling such motor events are interpreted. The frequency of the component is near the upper limit of a normal tremor frequency band. At the same time, it is impossible to regard the cyclic component in the motor command as a result of simple superposition of tremor on the targeted force development because the cyclic component is clearly synchronized with the force step initiation.