Transient changes in isotonic shortening velocity of frog rectus abdominis muscles in potassium contracture

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Abstract
Shortening velocities of the frog rectus abdominis muscle during isotonic releases in potassium contracture have been determined under various conditions of initial length, initial tension and load. The velocity of shortening under constant load at any particular muscle length is dependent upon the length and tension of the muscle before release; there is no unique force-velocity relation at constant length. An empirical formulation of the relations between shortening velocity and length is given. These results cannot be explained in terms of a simple model of the muscle in which there is an undamped elastic element in series with a contractile component whose shortening velocity is instantaneously determined by tension. Some possible explanations of the phenomena are discussed, and it is tentatively suggested that the transient condition following an isotonic release is dependent upon the rates of formation and breakage of cross-linkages between the actin and myosin filaments in the muscle.
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