The effect of hypoxia on pulmonary O2 uptake, leg blood flow and muscle deoxygenation during single‐leg knee‐extension exercise

Abstract

The effect of hypoxic breathing on pulmonary O₂ uptake (VO_2p), leg blood flow (LBF) and O₂ delivery and deoxygenation of the vastus lateralis muscle was examined during constant-load single-leg knee-extension exercise. Seven subjects (24 ± 4 years; mean ±s.d.) performed two transitions from unloaded to moderate-intensity exercise (21 W) under normoxic and hypoxic (P_ETO₂= 60 mmHg) conditions. Breath-by-breath VO_2p and beat-by-beat femoral artery mean blood velocity (MBV) were measured by mass spectrometer and volume turbine and Doppler ultrasound (VingMed, CFM 750), respectively. Deoxy-(HHb), oxy-, and total haemoglobin/myoglobin were measured continuously by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). VO_2p data were filtered and averaged to 5 s bins at 20, 40, 60, 120, 180 and 300 s. MBV data were filtered and averaged to 2 s bins (1 contraction cycle). LBF was calculated for each contraction cycle and averaged to 5 s bins at 20, 40, 60, 120, 180 and 300 s. VO_2p was significantly lower in hypoxia throughout the period of 20, 40, 60 and 120 s of the exercise on-transient. LBF (l min⁻¹) was approximately 35% higher (P > 0.05) in hypoxia during the on-transient and steady-state of KE exercise, resulting in a similar leg O₂ delivery in hypoxia and normoxia. Local muscle deoxygenation (HHb) was similar in hypoxia and normoxia. These results suggest that factors other than O₂ delivery, possibly the diffusion of O_2, were responsible for the lower O₂ uptake during the exercise on-transient in hypoxia.