Influence of frequency and duration of firing on the shape of potentials from different types of motor units in human muscles

Abstract

Changes in the potentials from separate motor units in human m. opponens pollicis in relation to contraction frequency and fatigue were studied. Higher frequency of tetanic stimulation produced shortening of the interval between onset of the potentials and their final positive maximum. The single-twitch time could be judged from this change. Stimulation with a frequency of 30 Hz for 3 min produced changes in the shape of the evoked potentials which were due to fatigue. These changes were mainly due to lengthening of the depolarized zone and were not uniform in the different motor units. Thus, three types of units could be differentiated: fast, resistant-to-fatigue; slow, resistant-to-fatigue; and fast, fatiguable. The fast, fatiguable motor units are large and have the highest velocity of propagation along the axon and muscle fibers; the slow, fatigue-resistant motor units are medium in size and have a lower velocity of propagation along the axon and muscle fibers; and the fast, resistant-to-fatigue motor units are small and have the lowest velocity of propagation along the axon and muscle fibers.