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Department of Physiology, Monash University, Clayton, Victoria, Australia.

1. The energetics of lengthening were studied in amphibian and mammalian skeletal muscle. The aims were to determine whether energy absorption during stretch is a general property of skeletal muscle and to investigate the influence of lengthening velocity on energy absorption. 2. Experiments were performed in vitro (21 degrees C) using bundles of muscle fibres from fast-twitch extensor digitorum longus and slow-twitch soleus muscles of the mouse and tibialis anterior muscles of a toad, Bufo marinus. Initial heat production and mechanical work done on muscles were measured during isovelocity lengthening. Enthalpy output during lengthening was calculated as the difference between the amount of heat produced and the work done. 3. For all three muscle types, more energy was put into muscles as work than was produced as heat. Thus, part of the energy put into muscles to stretch them must have been absorbed. 4. For all three muscle types, the amount of energy absorbed was constant at velocities exceeding approximately 0.5 Vmax (Vmax is the maximum shortening velocity), but was significantly lower at slow velocities of lengthening. The same amount of energy was absorbed by all three muscles when lengthened at > or = 0.5 Vmax. 5. It was concluded that absorption of energy during lengthening occurs in mammalian as well as amphibian muscle and that lengthening velocity has only a small effect on the amount of energy absorbed.