Power fatigue of the rat diaphragm muscle

We hypothesized that decrements in maximum power output ((W)over dot(max)) 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 degreesC) 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 (P-o) and W-max during fatigue. During each stimulus train, activation was isometric for the initial 360 ms during which P-o was measured; the muscle was then allowed to shorten at a constant velocity (30% V-max) for the final 40 ms, and (W)over dot(max) was determined. Compared with initial values, after 120 s of repetitive activation, P-o and (W)over dot(max) decreased by 75 and 73%, respectively. Maximum shortening velocity was measured in two ways: by extrapolation of the force-velocity relationship (V-max) and using the slack test [maximum unloaded shortening velocity (V-o)]. After 120 s of repetitive activation, V-max slowed by 44%, whereas V-o slowed by 22%. Thus the decrease in W with repetitive activation was dominated by force fatigue, with velocity fatigue playing a secondary role. On the basis of a greater slowing of V-max vs. V-o, we also conclude that force and power fatigue cannot be attributed simply to the total inactivation of the most fatigable fiber types.