A reexamination of the thermoelastic effect in active striated muscle.

Isolated Rana pipiens sartorius muscles at 0degreeC were stimulated via their nerves and small stretches or releases applied during the plateau of the isometric tetanus at lo. Extra heat above the isometric maintenance heat was produced during the drop in tension caused by release and, for very small releases (delta less than or equal to 0.5% lo), 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% lo less than or equal to deltal less than or equal to 3.0% lo. The mean heat:tension ratio R 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% lo less than or equal to deltal less than or equal to 3.0% lo the "contractile" component seems able to shorten about 1% lo without producing shortening heat.