Oxygen Consumption, Lactate Accumulation, and Sympathetic Response During Prolonged Exercise Under Hypoxia

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Abstract
Six men (33±3 years old) performed 1 h ergocycle exercise (60% V̇O2 max) at sea level and at a simulated altitude of 3000 m. A similar relative exercise intensity corresponded to a lower absolute work load (139±4 W) in hypoxic than normoxic (163±4 W) conditions. Lower oxygen uptake (V̇O2) with no change in ventilation (V̇E), respiratory exchange ratio (R), and heart rate (Hr) were observed during exercise under hypoxia compared to normoxia. A slow rise in V̇O2, after the initial 5 min exercise, was observed in normoxic (+230 ml/min) as well as in hypoxic (250 ml/min) conditions that might be, in part, related to oxidative removal of blood lactate. Peak blood lactate concentration reached at 30 min of exercise was similar in normoxia (4.5±0.4) and in hypoxia (4.7±0.5). However, while the lactate level decreased during exercise at sea level, it remained elevated throughout exercise in altitude. Blood lactate concentration measured at the end of exercise was significantly (P<0.05) higher in hypoxic (4.4±0.3) than in normoxic (3.2±0.4) conditions. Catecholamine response to exercise was similar in both conditions. We conclude that during prolonged exercise at a given relative work load, hypoxia does not affect cardiorespiratory and sympathetic responses but tends to increase blood lactate accumulation. Higher blood lactate concentrations during hypoxic exercise seems to reflect alterations in the removal of blood lactate rather than changes in glycolytic flux.