Abnormal use of remaining motor units during locomotion in peroneal palsy

The firing properties of single anterior tibial motor units were studied (during walking) with electromyogram (EMG) techniques in five normal subjects and in 10 patients who had footdrop due to an acute lesion of the common peroneal nerve. In the normal subjects, high threshold units did not fire in the ordinary step cycle and low threshold units fired at 10-25 Hz. In half of the paralyzed subjects, all motor units fired at 40-50 Hz for more than 200 msec in each step cycle, which could be sufficient for maximal force. In the other half of the paralyzed subjects, high threshold units were not recruited in each step cycle and low threshold units fired at 15-35 Hz, indicating that there were force reserves which were not used to prevent footdrop in each stride.