Voluntary motor commands in human ballistic movements.

The integrated electromyogram of the first dorsal interosseous muscle in normal humans presents a brief early burst in voluntary ballistic abduction of the index finger. Unexpected decrease in torque load does not change the burst. By contrast, in fast ramp abduction with a time-to-peak of 0.5 second, the electromyographic activity is fairly continuous up to the peak displacement, and it is silenced with a delay of 50 msec by unexpected unloading. Thus fast ramp, but not ballistic, voluntary contractions are under current long loop control from proprioceptive input. In a second set of experiments, the analysis of single motor unit potentials with highly selective electrodes in interosseous muscle indicated that ballistic thresholds are smaller than ramp thresholds under either isotonic or isometric conditions. However, the recruitment rank of the motor units was identical in finger abductions carried out in either ramp or ballistic modes.