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1 Dept of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: green{at}healthy.uwaterloo.ca.
In this study, we investigated the hypothesis that progressive cycle exercise to fatigue in normoxia (Norm) would result in a reduction in muscle contractility, membrane excitability and maximal Na+-K+-ATPase activity and that when the same exercise was performed in hypoxia (Hypox), the disturbances in these properties would be more emphasized. Ten untrained volunteers (age=20±0.37 yr and weight = 80.0±3.54 kg; x±SE) performed progressive exercise to fatigue on two occasions, namely while breathing normal room air (Norm; FIO2 = 0.21) and while breathing a normobaric hypoxic gas mixture (Hypox; FIO2 = 0.14). Muscle tissue samples were extracted from the vastus lateralis prior to exercise (Pre), at power outputs corresponding to approximately 50% and 70% of peak aerobic power (VO2peak), as determined in Norm, and at fatigue (Post) and analyzed for maximal Na+-K+-ATPase (K+-stimulated 3-O-methylfluorescein phosphatase) activity in homogenates. Exercise to fatigue in Norm resulted in a 32% reduction (P<0.05) in Na+-K+-ATPase activity (90.9±7.6 vs 62.1±6.4 nmol.mg protein-1.h-1). At fatigue, the reductions in Hypox (29%) were not different (81±5.6 vs 57.2±7.5 nmol.mg protein-1.h-1) from Norm. Measurement of quadriceps neuromuscular function, assessed prior to and following the exercise, indicated a generalized reduction (P<0.05) in maximal voluntary contractile force (MVC) and in force elicited at all frequencies of stimulation (10, 20, 30, 50, 100 Hz). In general, no differences were observed between Norm and Hypox. The properties of the compound action potential, namely amplitude, duration and area, which represent the electromyographic response to a single, supramaximal stimulus, were not altered by exercise or oxygen condition when assessed both during and after the progressive cycle task. It is concluded that progressive exercise, conducted in Hypox, results in an inhibition of Na+-K+-ATPase activity, reductions in MVC and force at different frequencies of stimulation, all of which are not different from observed with Norm. Moreover, these changes occur in absence of reductions in neuromuscular excitability.
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