AltitudeOmics: effects of 16 days acclimatization to hypobaric hypoxia on muscle oxygen extraction during incremental exercise

Abstract

Acute altitude exposure lowers arterial oxygen content ([Formula: see text]) and cardiac output ([Formula: see text]) at peak exercise, whereas O₂ extraction from blood to working muscles remains similar. Acclimatization normalizes [Formula: see text] but not peak [Formula: see text] nor peak oxygen consumption (V̇o_2peak). To what extent acclimatization impacts muscle O₂ extraction remains unresolved. Twenty-one sea-level residents performed an incremental cycling exercise to exhaustion near sea level (SL), in acute (ALT1) and chronic (ALT16) hypoxia (5,260 m). Arterial blood gases, gas exchange at the mouth and oxy- (O₂Hb) and deoxyhemoglobin (HHb) of the vastus lateralis were recorded to assess arterial O₂ content ([Formula: see text]), [Formula: see text], and V̇o₂. The HHb-V̇o₂ slope was taken as a surrogate for muscle O₂ extraction. During moderate-intensity exercise, HHb-V̇o₂ slope increased to a comparable extent at ALT1 (2.13 ± 0.94) and ALT16 (2.03 ± 0.88) compared with SL (1.27 ± 0.12), indicating increased O₂ extraction. However, the HHb/[Formula: see text] ratio increased from SL to ALT1 and then tended to go back to SL values at ALT16. During high-intensity exercise, HHb-V̇o₂ slope reached a break point beyond which it decreased at SL and ALT1, but not at ALT16. Increased muscle O₂ extraction during submaximal exercise was associated with decreased [Formula: see text] in acute hypoxia. The significantly greater muscle O₂ extraction during maximal exercise in chronic hypoxia is suggestive of an O₂ reserve.NEW & NOTEWORTHY During incremental exercise muscle deoxyhemoglobin (HHb) and oxygen consumption (V̇o₂) both increase linearly, and the slope of their relationship is an indirect index of local muscle O₂ extraction. The latter was assessed at sea level, in acute and during chronic exposure to 5,260 m. The demonstrated presence of a muscle O₂ extraction reserve during chronic exposure is coherent with previous studies indicating both limited muscle oxidative capacity and decrease in motor drive.

Keywords: NIRS; altitude; maximal exercise.