A B S T R A C T IsolatedRana pipiens sartorius muscles at 0°C were stimulated via their nerves and small stretches or releases applied during the plateau of the isometric tetanus at to. Extra heat above the isometric maintenance heat was produced during the drop in tension caused by release and, for very small releases (A-0.5% to), was completely reabsorbed during tension recovery. The extra heat was always directly proportional to the tension change. Heat absorption proportional to the tension change was also observed during the increase in tension produced by small stretches in the range 0.5% t0 -< At ~ 3.0% to. The mean heat:tension ratioR in seven experiments was -0.0084, which is within the range of values reported previously by Woledge. In addition, it was found that during tension recovery after small releases of 1.0% to -< At <_ 3.0% t0 the "contractile" component seems able to shorten about 1% #0 without producing shortening heat.
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, 10, the maximum lengthwith 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, l _< 10, for an isotonically contracting muscle, not only is the force-velocity relation valid for the initial reference length, 10, but also for any other length. The analysis in this report indicates that the constants a/Po and b/lo remain fixed throughout the length change of afterloaded isotonic shortening in the Rana pipiens sartorius muscles. Fenn and Marsh (1935) established the " F e n n effect," Hill suggested that the isotonic force-velocity equation of the striated muscle could be described accurately in terms of a simple hyperbolic relation:
I N T R O D U C T I O N 3 yr afterwhere a and b are constants and P0 equals isometric tension at 10. Hill achieved this result by means of study involving heat measurements in the frog sartorius muscles during contraction (A. V. Hill, 1938).Independent of heat data, the force-velocity relation can also be derived from mechanical measurements. This characteristic relation found for mechanical movement, though differing in the time factor, is similar in character II2 5
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