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 (CAIP) (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 l · min^-1 vs 1.96 ± 0.42 I · min^-1, p < 0.01) whereas extraventilatory VO₂ was not significantly different (1.80 ± 0.441 min^-1 vs 1.77 ± 0.361 min^-1). During CRP, the slope relating VO₂ to power output computed from the three work lods was not statistically different between normoxia and hiypoxia (ΔVO₂/Δw = 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 (ΔVO₂/Δw) 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.

Key words

Metabolic efficiency - normobaric acute hypoxia - ventilation - work of breathing