Effect Of Severe Hypoxia On Endurance Capacity And Quadriceps Muscle Fatigue In Healthy Humans

Abstract

Severe hypoxia elicits a significant decrease in whole body endurance capacity. It is unclear whether this decrease in endurance capacity is accompanied by an increase in the severity of exercise-induced locomotor muscle fatigue. PURPOSE To determine the effect of severe hypoxia on endurance capacity and quadriceps muscle fatigue. METHODS On separate occasions (order randomised), 9 male subjects (mean ± SEM VO2peak= 56.5 ± 2.7 ml/kg/min) cycled to exhaustion at > 90% VO2peak under conditions of normoxia (2peakFI,O2= 0.21; NORM-EXH) and acute hypoxia (2peakFI,O2= 0.13; HYPOX). The subjects also exercised in normoxia for a time equal to that achieved during hypoxia (NORM-CTRL). Quadriceps twitch force (Qtw), in response to supramaximal paired magnetic stimuli of the femoral nerve (1–100 Hz), was assessed pre- and up to 70 min post-exercise. RESULTS End-exercise measurements are contained in table.TableCONCLUSIONS Severe hypoxia affects performance during heavy-intensity exercise in part because of the direct effect of decreased systemic O transport on the force output of the limb locomotor muscles, shown by the reduced twitch force after hypoxia vs. normoxia at exercise iso-time. An additional effect of severe hypoxemia on exercise capacity was not dependent upon end-organ limb muscle fatigue, as implicated by the finding that the reduction in force output after hypoxic exercise to exhaustion was only about one-half that observed after exercise to exhaustion in normoxia. This comparison implies that reflex-induced inhibition of motor output to locomotor muscles may have occurred during exercise in severe hypoxia. Supported by NHLBI.