The effect of shortening velocity on the shortening heat and its relationship to the distance shortened.

Irving and Woledge showed using isovelocity (1/2 Vmax) releases that the shortening heat per unit shortening declines with distances shortened. This non-linear behavior of the shortening heat was examined in experiments in which muscles shortened during isovelocity releases at 1/4, 1/2, and 1/1 Vmax. As shortening velocity increases, the reduction in shortening heat production per unit shortening with distance becomes more pronounced. These results could be a consequence of the facts that shortening heat production is load dependent and during shortening at constant velocity, the average force exerted by the muscle shortening a small distance is greater than that exerted by the muscle shortening greater distances. To test this idea, shortening heat production was measured in releases in which the force remained constant during constant velocity shortening. Again, the shortening heat produced per unit shortening declined with distance shortened, and the effect at Vmax was more pronounced than at 1/2 Vmax. These results agree with the two state crossbridge model proposed by Irving and Woledge and suggest that: 1) significant lengths of time are required (minimum of 50 ms at Vmax and 90 ms at 1/2 Vmax) for the energy liberation to reach a steady state in the transition from isometric to shortening contractions, 2) shortening induces shifts in the distribution of crossbridge states whose magnitude increases with shortening velocity, and 3) the non-linearity of shortening heat production is related to the energy imbalance observed during rapid shortening.