The after-effects of repetitive stimulation on the isometric twitch contraction of rat fast skeletal muscle.

1. The peak tension and time course of isometric twitch contractions of rat extensor digitorum longus muscle in vitro (35 degrees C) have been measured at various stages of potentiation following repetitive stimulation at 20 c/s and 300 c/s.2. Potentiation of the peak twitch tension increases with an increase in the number of repetitive stimuli up to a maximal level of about 1.9 times the control value. The relation between potentiation and numbers of stimuli is dependent on the frequency of stimulation.3. Potentiation of peak twitch tension is maximal shortly after the end of repetitive stimulation and subsequently decays exponentially at a rate which is dependent on the number of stimuli in the train and the frequency of stimulation.4. Short trains of stimuli bring about nearly maximal potentiation with little or no change in contraction time and a small decrease in half-relaxation time.5. Long trains of stimuli increase the contraction time, the half-relaxation time and the twitch duration in addition to potentiating the peak tension. The changes in twitch time course are dependent on the number of repetitive stimuli and the frequency of stimulation.6. The results are discussed in relation to possible mechanisms of post-tetanic potentiation and the degree of activation of mammalian and amphibian muscle fibres.