The kinetics of heat production in response to active shortening in frog skeletal muscle.

1. Tension and heat production were measured at 0 degree C in sartorius muscles from Rana temporaria in response to two extents of shortening at five velocities. Shortening was from approximately 2.4 to 2.2 microns, 2.4 to 2.3 microns and 2.3 to 2.2 micron at velocities per half-sarcomere from 0.2 to 1.56 micron s-1. 2. Tension became approximately constant at all velocities. Records of heat rate obtained by differentiating traces from which thermoelastic heat had been subtracted became negative early in shortening and then rose. Heat rate became constant during shortening only at the lowest velocity and was still rising at the end of shortening at higher velocities. The highest heat rate occurred at the end of shortening at the two highest velocities. At the end of shortening at all velocities heat rate gradually approached the isometric level measured at the short length, the half-time for decline being largest following the slowest larger shortening. 3. Heat produced as a consequence of shortening but not associated with tension recovery was determined by subtracting shortening heat measured in response to two extents of shortening to the same muscle length. The differences in shortening heat continued to increase after shortening ended, and more of the extra heat produced in response to shortening appeared after the end of rapid shortening than during shortening itself. 4. Shortening heat coefficients calculated in different ways were similar to coefficients determined in previous studies. Coefficients calculated from measurements that excluded heat produced by tension recovery and allowed for continued production of heat by processes initiated by shortening were found to increase linearly with the force maintained during shortening. 5. The results show that the kinetics of heat production during and after shortening are very sensitive to the speed of shortening and that steady rates of energy liberation are not attained during shortening of less than or equal to 10% of muscle length at velocities greater than or equal to 12% of maximum velocity.