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J Appl Physiol 89: 2215-2219, 2000;
8750-7587/00 $5.00
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Vol. 89, Issue 6, 2215-2219, December 2000

Power fatigue of the rat diaphragm muscle

Bill T. Ameredes1, Wen-Zhi Zhan2, Y. S. Prakash2, Rene Vandenboom2, and Gary C. Sieck2,3

1 Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261; and Departments of 2 Anesthesiology and 3 Physiology and Biophysics, Mayo Clinic and Mayo Medical School, Rochester, Minnesota 55905

We hypothesized that decrements in maximum power output (Wmax) of the rat diaphragm (Dia) muscle with repetitive activation are due to a disproportionate reduction in force (force fatigue) compared with a slowing of shortening velocity (velocity fatigue). Segments of midcostal Dia muscle were mounted in vitro (26°C) and stimulated directly at 75 Hz in 400-ms-duration trains repeated each second (duty cycle = 0.4) for 120 s. A novel technique was used to monitor instantaneous reductions in maximum specific force (Po) and Wmax during fatigue. During each stimulus train, activation was isometric for the initial 360 ms during which Po was measured; the muscle was then allowed to shorten at a constant velocity (30% Vmax) for the final 40 ms, and Wmax was determined. Compared with initial values, after 120 s of repetitive activation, Po and Wmax decreased by 75 and 73%, respectively. Maximum shortening velocity was measured in two ways: by extrapolation of the force-velocity relationship (Vmax) and using the slack test [maximum unloaded shortening velocity (Vo)]. After 120 s of repetitive activation, Vmax slowed by 44%, whereas Vo slowed by 22%. Thus the decrease in Wmax with repetitive activation was dominated by force fatigue, with velocity fatigue playing a secondary role. On the basis of a greater slowing of Vmax vs. Vo, we also conclude that force and power fatigue cannot be attributed simply to the total inactivation of the most fatigable fiber types.

fatigability; isometric; isovelocity; slack test


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