NATURAL AND ARTIFICIAL ACTIVATION OF MOTOR UNITS—A COMPARISON

Abstract

The activation of motor units by voluntary innervation and by electrical stimulation of the motor nerve in man has been studied in the first dorsal interosseous and the peroneus longus muscles. A special technique involving stimulation of the motor nerve with linearly rising currents was used, which makes a separate study of motor units of different thresholds and spike sizes possible. The recruitment of motor units to stimulation with very slowly rising currents was compared with that during sustained voluntary contractions. It was found that both types of contraction start with a unit of small amplitude followed by units of progressively larger spike size. The units appearing in a particular order during a voluntary contraction seem to be identical with those activated in the same order by electrical stimulation. The initial discharge frequencies of identical units at threshold strength in both forms of contraction were found to be the same and the increase of frequency during increased contraction by a given unit when the next unit in the sequence appears is also the same in both cases.