EFFECTS OF FATIGUE ON FORCE-VELOCITY RELATION OF DIAPHRAGM - ENERGETIC IMPLICATIONS

Characteristics of the force (P)-velocity (V) relationship were invest igated before and after fatigue in isolated diaphragm muscle of two sp ecies: mouse (n = 12), in which diaphragm is composed almost exclusive ly of fast-twitch fatigue-resistant fiber type, and rat (n = 12), in w hich diaphragm contains both fatigue-sensitive and fatigue-resistant f iber types. Following fatigue protocol, the mechanical performance of both the contraction and relaxation phases were significantly depresse d in the two species. Consequences of the fatigue procedure on P-V cha racteristics differed markedly according to the two species. In mouse diaphragm, fatigue trial did not change the G curvature of the P-V hyp erbola, the maximum mechanical efficiency Eff(max), or the normalized peak power output M(2). Conversely, in rat diaphragm, fatigue resulted in higher G curvature (p < 0.001), higher Eff(max) (p < 0.001) and a lower M(2) (p < 0.001) compared with control values. This suggests tha t fatigue improved the economy of force generation in mixed rat diaphr agm and not in mouse. This is consistent with an increased proportion of slow fatigue-resistant fibers caused by fatigue of fast fatigue-sen sitive fibers in rat diaphragm.