Cessation of human motor unit discharge during sustained maximal voluntary contraction.

The purpose of this study was to determine whether cessation of motor unit discharge contributes to fatigue in human subjects. Multiple fine-wire and tungsten microelectrodes were inserted into the extensor digitorum or extensor indicis muscles of the forearm in an attempt to record the activity of the same motor unit from different locations within either muscle while subjects maintained a maximal voluntary contraction of the finger extensors until force dropped by approximately 50%. The activities of 13 motor units were followed for extended periods during the fatigue task. Of these, six appeared to cease discharging prior to the end of the task, which could not be attributed to electrode movement. These findings suggest that some motor neurons may not be able to discharge continuously in the presence of sustained volitional synaptic drive or that excitatory drive may diminish during maximal voluntary effort.