The influence of stimulus parameters on contractions of isolated frog muscle fibres.

1. Reversible changes in twitch and tetanus contractions of isolated frog muscle fibres were produced by varying the duration and strength of a transversely applied d.c. stimulus.2. When the stimulus strength was at least 1.1 times threshold the peak force of the contraction elicited was reduced by stimulus durations longer than 1 msec at 20 degrees C, and by durations longer than 3 msec at 2-5 degrees C.3. As the strength of a prolonged stimulus was progressively increased to 3 times rheobase or more, peak twitch and tetanus force declined to 40-70% of the amplitude with short (0.2-0.5 msec) stimuli. The time to peak of the twitch was slightly decreased and the first phase of relaxation was accelerated.4. Cine-micrography of fibres which were permitted to shorten under a light load revealed that the side of the fibre adjacent to the anode was unactivated under conditions which also reduced peak force output.5. With stimulus durations longer than 20 msec, peak force, time to peak, and relaxation time increased as the stimulus strength increased from 2.5 to 3 times rheobase. This might have been produced by a cathodal contracture following the action potential.6. The results are discussed in relation to previous studies of activation in striated muscle.