The effect of length range on heat rate and power during shortening nearin situ length in frog muscle

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Abstract
Heat rate and tension were measured during the steady state of isovelocity shortening in frog sartorius muscles at two speeds (7.5 and 2.0 mm s−1) through two ranges of the length-tension curve: 1.11 to 1.01L 0and 1.01 to 0.91L 0. Both heat rate and mechanical power were higher at short length than at long length. The ratio of mechanical power to total energy rate was smaller at short length. The difference in heat rates in the two length ranges was greatest during shortening at 7.5 mm s−1 and least during isometric contractions. Calculations were made for two extreme interpretations of the results, based on the assumption that 30% of the isometric maintenance heat rate at optimum length is produced by processes related to activation: (1) that all the difference in heat rate results from effects of length on activation processes and (2) that all the difference results from effects on contractile processes. The rate of heat production by activation processes would have to be 1.5 times higher at short than at long length in an isometric contraction, 1.7 times higher during shortening at 2 mm s−1 and 2.0 times higher at 7.5 mm s−1 to explain the differences in heat rate. The rate of contractile processes would have to be only 1.2 times higher at short than at long length. The results favour the interpretation that the contractile process itself is sensitive to sarcomere length near the plateau of the length-tension curve. The rise in heat rate above the isometric level at 2 mm s−1 did not occur immediately with the beginning of shortening and lagged behind the establishment of steady tension by a few hundred milliseconds. This result indicates that the biochemical steady state is also delayed during shortening.