Effects of Environmental Conditions on Recruitment and Peripheral Fatigue

Includes bibliographical references (pages 25-52).We investigated the effects of moderate hyperthermia and hypoxia on the relative contributions on recruitment and peripheral fatigue to exercise intolerance. Five low-fit individuals (men VO[subscript ???max] [less than or equal to]40 ml/kg/min, women VO[subscript ???max] [les than or equal to]34 ml/kg/min) performed constant-load exercise until intolerance in three different environmental conditions; normoxia, moderate hypoxia (F[subscript IO???] = 0.15), and moderate hyperthermia (42 ??C, 40% RH). Prior to each trial, a linear relationship between power and EMG at various efforts was established (R?? = 0.98). At the point of intolerance, subjects performed 5-6 pedal strokes at maximal effort to determine fatigue. Time to intolerance was shorter in the hyperthermic trials (151.4 ?? 51.4 s) compared to the normoxic (269.6 ?? 62.2 s, p = 0.001) and hypoxic (222.4 ?? 34.6 s, p = 0.008) trials. However, there were no differences in performance fatigue (-189.2 ?? 111.9 W, -205 ?? 95.3 W, -180.8 ?? 108.5 W) peripheral fatigue (-94.2 ?? 74.4 W, -107.2 ?? 50.7 W, -92.8 ?? 71.7 W) or recruitment activity (-23.4 ?? 13.1%, -25.3 ?? 13.8%, -26.5 ??14.5%) between normoxic, hypoxic, and hyperthermic conditions, respectively. With decreased times to intolerance and similar levels of peripheral fatigue, the adverse conditions increased the rate of peripheral fatigue. As peripheral fatigue has been considered to govern central fatigue, central fatigue was also unaffected by the conditions. Performance fatigue, being the result of both central and peripheral factors, also remained unchanged. These results indicated moderate hyperthermia and hypoxia do not alter central or peripheral fatigue contributions towards intolerance. Additionally, these results support the concept of an individualized critical peripheral fatigue threshold. Once reached, this threshold elicits recruitment fatigue, restricting further increases in peripheral fatigue possibly to prevent muscle/organ damage/failure