The effect of the performance of work on total energy output and metabolism during muscular contraction.

1. The production of heat (h) and work (w) and the changes in phosphocreatine (PCr) and ATP have been measured on tetanized isolated frog muscles (unpoisoned and in oxygen at 0 degrees C) during shortening at constant velocity and during isometric contraction (both without relaxation). The former type of contraction was designed to maximize the fraction w/(h + w); the latter to minimize it.2. The duration of the isometric contraction was made considerably longer than that of the isovelocity contraction so that the (h + w) productions during the two contractions were approximately equal.3. The PCr break-down during the working contraction was considerably greater than that during the isometric contraction.4. No detectable ATP changes occurred.5. The break-down of PCr is sufficient to account for the work evolved: there is no reason to suppose that the work comes from an unidentified source.6. In both types of contraction extra energy is evolved that cannot be accounted for by concurrent splitting of PCr. The time course of evolution of this extra energy is similar in all types of contraction, suggesting that it may arise from a process other than cross-bridge interaction.7. The results are discussed in terms of current cross-bridge theory and muscle kinetics. The mean cycle times of a cross-bridge during working and isometric contractions are 0.12 sec and 0.34 sec respectively. During the working contraction cross-bridges spend about one quarter of the time attached to actin filaments.