The dependence of the twitch course of medial gastrocnemius muscle of the rat and its motor units on stretching of the muscle

Abstract

The relationship between the force of a single twitch of the medial gastrocnemius muscle of the rat and contraction and half-relaxation times, on one hand, and the load of the muscle on the other, was studied. Twitches of the whole muscle and its individual motor units were induced. The optimal load, at which the majority of motor units reached the greatest twitch force, was 10 G. Mean optimal loads for twitches of different types of motor units were very similar. Slow motor units reached a slightly greater twitch force at greater loads (12.5 G) than at 10 G. However, the optimal load for the twitch of the whole muscle was much greater. It was 47 G on the average. The contraction and half-relaxation times of motor units, as well as of the whole muscle, became longer as the force stretching the muscle increased. Half-relaxation time changed more rapidly than contraction time. Both parameters were undergoing the greatest changes in slow motor units.