Firing rate and recruitment order of toe extensor motor units in different modes of voluntary conraction.

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Abstract
The discharge properties of individual motor units in different modes of voluntary contraction were studied with electromyographic techniques in the short toe extensor muscle of normal man. The short toe extensor muscle consisted of type I and type II muscle fibers in about equal proportion. In some subjects there was type-grouping so that recordings with sufficient selectivity could easily be obtained. Certain motor units could be driven continuously, attained regular firing intervals even at a firing rate of 10/s, increased slowly in firing rate with increase in contraction strength, had maximum firing rate below 30/s during sustained contraction but above 60/s in twitch contraction. Other motor units could not be driven continuously, did not fire repeatedly at rates below 20/s, increased rapidly in firing rate with increase in contraction strength and attained firing rates above 100/s. There were intermediate forms between continuously firing low frequency motor units and intermittently firing high frequency motor units. In a prolonged contraction of constant strength only continuously firing motor units were active. On rapid accelerations, continuously and intermittently firing motor units were active and played about the same role. This applied to prolonged series of accelerations as in rhythmically alternating movements. In twitch contractions selective activation of intermittently firing motor units occurred if the muscle was relaxed prior to the twitch and great effort was used to elicit the twitch and minimum duration of the twitch was intended. Apparently, continuously firing low frequency motor units have type I muscle fibers and intermittently firing high frequency units have type II muscle fibers; the order of recruitment and the relative roles of the 2 motor unit types are probably adapted to the mode of contraction.