Variability of the interimpulse intervals of motor units upon voluntary activation.

Twenty subjects were investigated for voluntary control of separate motor units (MU) in the biceps brachii muscle under visual and auditory feedback. After 20-fold repetition of the task for sustaining stable firing frequency it turned out that in every trial the frequency was different and the variability of the interimpulse intervals changed in an unexpected way. It increased with the lengthening of the interimpulse intervals, but in an uneven manner. There appeared a decrease of the variability at steps of about 20 ms. A supraspinal excitatory input, rhythmically acting, is supposed to be the cause of this unevenness.