Considerable attention has been directed to the characteristic force-velocity relation discovered by A. V. Hill in the study of muscle kinematics. Models of contractile process were tested on the basis of their compatibility with the Hill equation. However, almost all the isotonic data have been restricted to one length, l0, the maximum length with almost no resting tension; the velocities measured are those initial values when the load begins to move. The force-velocity curve extrapolates to zero velocity for isometric tension, but only for the tension at that one length. Very few efforts have been made to study the profiles of the curves throughout the range of lengths over which shortening takes place. In examining the length region, ll0, for an isotonically contracting muscle, not only is the force-velocity relation valid for the initial reference length, l0, but also for any other length. The analysis in this report indicates that the constants a/P0 and b/l0 remain fixed throughout the length change of afterloaded isotonic shortening in the Rana pipiens sartorius muscles.

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