Firing properties of single human motor units during locomotion.

The discharge properties of single motor units in the short extensors of the toes were studied during normal locomotion using electromyographic recordings which permitted the identification of single motor unit potentials. The single motor units were recruited in the same order during slowly increasing speed of locomotion as they were during slowly increasing voluntary tension, i.e. motor units with low axonal conduction velocity were recruited before motor units with higher conduction velocity. Low threshold units fired five to ten times at 80-40 ms intervals in each step cycle during walking at normal speed. Motor units with moderately high thresholds fired once or only a small number of times in each cycle at shorter interspike intervals. High threshold motor units did not participate in the step cycle during walking at normal speeds but fired in short high-frequency bursts in corrective movements.