Oxygen uptake during submaximal incremental and constant work load exercises in hypoxia.

The effect of acute hypoxia on oxygen uptake (VO2) was studied during incremental (IE) and constant work load exercises. Twenty-two healthy subjects performed two incremental exercises on a bicycle ergometer under normoxic (21% O2) and hypoxic (10.4% O2) conditions. Fifteen subjects performed a constant work load exercise at the same absolute power (CAP) (116 +/- 33 W), while seven other subjects performed three constant work load exercises at the same relative power (CRP) (50, 60 and 70% of VO2max) in both conditions. VO2 was defined as extraventilatory when the estimation of respiratory muscles O2 consumption was subtracted from the total VO2. During IE, the slope of the linear regression relating VO2 to work rate was higher in normoxia than in hypoxia (11.6 +/- 1.2 ml.l-1.W-1 vs 10.1 +/- 1.1 ml.l-1.W-1, p < 0.01). During CAP, VO2 was lower in normoxia than in hypoxia (1.88 +/- 0.45).min-1 vs 1.96 +/- 0.42 l.min-1, p < 0.01) whereas extraventilatory VO2 was not significantly different (1.80 +/- 0.441.min-1 vs 1.77 +/- 0.36) l.min-1). During CRP, the slope relating VO2 to power output computed from the three work loads was not statistically different between normoxia and hypoxia (delta VO2/delta w = 11.9 +/- 3.1 ml.min-1.W-1 vs 12.3 +/- 1.2 ml.min-1.W-1). These findings showed that during CRP, the metabolic efficiency (delta VO2/delta W) was the same in normoxia and in hypoxia. During CAP, the respiratory muscles O2 consumption might have accounted for the difference in VO2 consumption between hypoxia and normoxia.