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 d isproportionate 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 40 0-ms-duration trains repeated each second (duty cycle = 0.4) for 120 s. A n ovel technique was used to monitor instantaneous reductions in maximum spec ific force (P-o) and W-max during fatigue. During each stimulus train, acti vation 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) f or the final 40 ms, and (W)over dot(max) was determined. Compared with init ial values, after 120 s of repetitive activation, P-o and (W)over dot(max) decreased by 75 and 73%, respectively. Maximum shortening velocity was meas ured in two ways: by extrapolation of the force-velocity relationship (V-ma x) and using the slack test [maximum unloaded shortening velocity (V-o)]. A fter 120 s of repetitive activation, V-max slowed by 44%, whereas V-o slowe d by 22%. Thus the decrease in W with repetitive activation was dominated b y force fatigue, with velocity fatigue playing a secondary role. On the bas is of a greater slowing of V-max vs. V-o, we also conclude that force and p ower fatigue cannot be attributed simply to the total inactivation of the m ost fatigable fiber types.