Total oxygen consumption following contraction at controlled shortening velocities was determined for ten twitches of frog sartorius muscle. Under the conditions of this investigation maximum oxygen consumption occurred at approximately 30 mm/sec. shortening velocity and decreased for larger and smaller velocities. Mechanical efficiency was calculated and found to show a maximum at approximately 20 mm/sec. shortening velocity. The relationship between this study and a similar investigation in which heat changes are measured as a function of shortening velocity is discussed.

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