Tension minima and activation in frog twitch muscle fibres

Responses to restimulation at different times during isometric relaxation were recorded from single muscle fibres at 15 degrees C. The reactivation latency was constant over a wide range of tensions but the time (tmin) between restimulation and the subsequent tension minimum always varied by several milliseconds. Variation in tmin was directly correlated with the rate of relaxation at the time of restimulation and was inversely related to tension throughout most of the phase of relaxation. The results are discussed in relation to excitation-contraction coupling and it is shown that the tension minima did not collectively represent the time course of activation of the sarcomere.