The dynamic characteristics of the rat gracilis anticus muscle at 17.5°C have been determined by isotonic and isometric loading. For a fixed initial length these characteristics were represented either as a family of lengthvelocity phase trajectories at various isotonic afterloads or as a series of forcevelocity curves at different lengths. An alternate method of viewing these data, the length-external load-velocity phase space, was also generated. When the muscle was allowed to shorten from different initial lengths, the velocity of shortening achieved at a given length was lower for longer initial lengths. The amount of departure was also dependent upon the isotonic load, the greater the load the greater the departure. The departures were not caused by changes in the elastic elements of the muscle or fatigue in the ordinary sense. When the behavior of the muscle was investigated at different frequencies of stimulation, the shortening velocity was a function of the number of stimulating pulses received by the muscle at a given frequency. The shortening velocity of the rat gracilis anticus muscle is, therefore, not only a function of load and length, but also of an additional variable related to the time elapsed from onset of stimulation. I N T R O D U C T I O NThe function of skeletal muscle is to contract, and in so doing to exert a force against its environment. The relation between the force the muscle can exert and the length of the muscle when it is stimulated tetanically under isometric conditions has been expressed by the length-tension diagram. When a muscle is permitted to shorten during stimulation, the velocity with which it begins to shorten is a function of the load imposed on the muscle. The familiar force-velocity curve, first described by Fenn and Marsh (1) and Hill (2), describes this relation. However, there is a family of force-velocity curves, depending on the length from which shortening begins. The relation between velocity and length throughout the complete excursion from initial to final length has been displayed by Carlson (3) for frog sartorins muscle and by 369

A new technique is proposed for computing the active state of striated muscle, based on the three component model of Fenn and Marsh (8) and of Hill (7). The method permits calculation of the time course of the active state from its peak to the time at which maximum isometric twitch tension is reached. The intormation required for the calculation can be obtained from a single muscle without moving it from its mount in the lever system. The time course of the active state proved to be a function of the length of the muscle. This length dependency led to the predictions that (a) the length at which maximum force is developed during tetanic stimulation is different from that at which it is developed during a twitch, and (b) the tetanus-twitch tension ratio is a function of length. Both predictions were verified in a series of experiments on the rat gracilis anticus muscle at 17.5°C.

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