Frog Fast Muscle: I. Mechanical Power Output in Isotonic Twitches

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Abstract
The paper is primarily concerned with predicting the response of the muscle (frog ilio-fibularis fast fibres) to a given stimulus under given conditions of load. The predictive value of theories based on ‘active state ‘is considered in this context. A method is described for measuring the speed of shortening at frequent intervals during an isotonic twitch. A family of speed vs. tension curves is drawn up, each curve representing a particular time after the stimulus. These are translated into power vs. tension curves. The rate of fall of mechanical power output increases linearly with time after the stimulus, but is independent of tension. At the beginning of the twitch, mechanical power output increases slightly with temperature, but later in the twitch the trend is reversed, as rate of fall of power also increases with temperature. The high extensibility, characteristic of resting muscle, reappears suddenly at the end of an isotonic twitch, causing a ‘break’ in the speed record. This occurs earlier at low than at high tensions. The basis of predicting an isometric twitch from isotonic data, based on certain assumptions which have been made by previous authors, is explained. The tension peak in an isometric twitch is reached sooner than would be predicted on these assumptions.