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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Inspiratory muscle work in acute hypoxia influences locomotor muscle fatigue and exercise performance of healthy humans

John Rankin Laboratory of Pulmonary Medicine, University of Wisconsin Medical School, Madison, Wisconsin

Submitted 22 June 2007 ; accepted in final form 17 August 2007

Our aim was to isolate the independent effects of 1) inspiratory muscle work (W_b) and 2) arterial hypoxemia during heavy-intensity exercise in acute hypoxia on locomotor muscle fatigue. Eight cyclists exercised to exhaustion in hypoxia [inspired O₂ fraction (FI_O2) = 0.15, arterial hemoglobin saturation (Sa_O2) = 81 ± 1%; 8.6 ± 0.5 min, 273 ± 6 W; Hypoxia-control (Ctrl)] and at the same work rate and duration in normoxia (Sa_O2 = 95 ± 1%; Normoxia-Ctrl). These trials were repeated, but with a 35–80% reduction in W_b achieved via proportional assist ventilation (PAV). Quadriceps twitch force was assessed via magnetic femoral nerve stimulation before and 2 min after exercise. The isolated effects of W_b in hypoxia on quadriceps fatigue, independent of reductions in Sa_O2, were revealed by comparing Hypoxia-Ctrl and Hypoxia-PAV at equal levels of Sa_O2 (P = 0.10). Immediately after hypoxic exercise potentiated twitch force of the quadriceps (Q_tw,pot) decreased by 30 ± 3% below preexercise baseline, and this reduction was attenuated by about one-third after PAV exercise (21 ± 4%; P = 0.0007). This effect of W_b on quadriceps fatigue occurred at exercise work rates during which, in normoxia, reducing W_b had no significant effect on fatigue. The isolated effects of reduced Sa_O2 on quadriceps fatigue, independent of changes in W_b, were revealed by comparing Hypoxia-PAV and Normoxia-PAV at equal levels of W_b. Q_tw,pot decreased by 15 ± 2% below preexercise baseline after Normoxia-PAV, and this reduction was exacerbated by about one-third after Hypoxia-PAV (–22 ± 3%; P = 0.034). We conclude that both arterial hypoxemia and W_b contribute significantly to the rate of development of locomotor muscle fatigue during exercise in acute hypoxia; this occurs at work rates during which, in normoxia, W_b has no effect on peripheral fatigue.

work of breathing; arterial oxygen content; altitude; limb blood flow; expiratory flow limitation

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