Changes in motoneurone firing rates during sustained maximal voluntary contractions.

Abstract

W microelectrodes were used to record the electrical activity of single motor units in the human adductor pollicis during maximal voluntary contractions. The potentials were characteristic of those from single muscle fibers. In brief maximal contractions, the firing rates of over 200 motor units were obtained from 5 normal subjects. Four subjects had a similar range (mean 26.4 .+-. 6.5 Hz), while the 5th was slightly higher (35 .+-. 7.4 Hz). When maximal voluntary force was sustained for 40-120 s, there was a progressive decline in the range and mean rate of motor unit discharge. In the first 60 s, mean rates fell from about 27 Hz-15 Hz. Evidently, those units with the highest initial frequencies changed rate most rapidly. Evidently, this decline in motor unit discharge rates is not responsible for force loss, but it may enable effective modulation of voluntary strength by rate coding to continue during fatigue.