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SLEEP AND TEMPERATURE REGULATION
1Centre for Sports Medicine and Human Performance, Brunel University, Middlesex, United Kingdom; 2John Rankin Laboratory of Pulmonary Medicine, Department of Population Health Sciences, University of Wisconsin, Madison, Wisconsin; and 3Vermont Lung Center, College of Medicine, University of Vermont, Burlington, Vermont
Submitted 20 April 2006 ; accepted in final form 26 August 2006
We hypothesized that severe hypoxia limits exercise performance via decreased contractility of limb locomotor muscles. Nine male subjects [mean ± SE maximum O2 uptake (
O2 max) = 56.5 ± 2.7 ml·kg–1·min–1] cycled at 
90% O2 max to exhaustion in normoxia [NORM-EXH; inspired O2 fraction (FIO2) = 0.21, arterial O2 saturation (SpO2) = 93 ± 1%] and hypoxia (HYPOX-EXH; FIO2 = 0.13, SpO2 = 76 ± 1%). The subjects also exercised in normoxia for a time equal to that achieved in hypoxia (NORM-CTRL; SpO2 = 96 ± 1%). Quadriceps twitch force, in response to supramaximal single (nonpotentiated and potentiated 1 Hz) and paired magnetic stimuli of the femoral nerve (10–100 Hz), was assessed pre- and at 2.5, 35, and 70 min postexercise. Hypoxia exacerbated exercise-induced peripheral fatigue, as evidenced by a greater decrease in potentiated twitch force in HYPOX-EXH vs. NORM-CTRL (–39 ± 4 vs. –24 ± 3%, P < 0.01). Time to exhaustion was reduced by more than two-thirds in HYPOX-EXH vs. NORM-EXH (4.2 ± 0.5 vs. 13.4 ± 0.8 min, P < 0.01); however, peripheral fatigue was not different in HYPOX-EXH vs. NORM-EXH (–34 ± 4 vs. –39 ± 4%, P > 0.05). Blood lactate concentration and perceptions of limb discomfort were higher throughout HYPOX-EXH vs. NORM-CTRL but were not different at end-exercise in HYPOX-EXH vs. NORM-EXH. We conclude that severe hypoxia exacerbates peripheral fatigue of limb locomotor muscles and that this effect may contribute, in part, to the early termination of exercise.
hypoxemia; muscle fatigue; exercise performance
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