Differential kinetics of the cardiac, ventilatory, and gas exchange variables during walking under moderate hypoxia

We investigated the effects of moderate hypoxia (FiO2 = 15%) on different kinetics between pulmonary ventilation ([Formula: see text]) and heart rate (HR) during treadmill walking. Breath-by-breath [Formula: see text], oxygen uptake ([Formula: see text]), carbon dioxide output ([Formula: see text]), and HR were measured in 13 healthy young adults. The treadmill speed was sinusoidally changed from 3 to 6 km·h-1 with four oscillation periods of 1, 2, 5, and 10 min. The amplitude (Amp), phase shift (PS) and mean values of these kinetics were obtained by harmonic analysis. The mean values of all of these responses during walking at a sinusoidally changing speed became greater under hypoxia compared to normoxia (FiO2 = 21%), indicating that moderate hypoxia could achieve an increased energy expenditure (increased [Formula: see text] and [Formula: see text]) and hyperventilation. The Amp values of the [Formula: see text], [Formula: see text], and [Formula: see text] kinetics were not significantly different between normoxia and hypoxia at most periods, although a significantly smaller Amp of the HR was observed at faster oscillation periods (1 or 2 min).The PS of the HR was significantly greater under hypoxia than normoxia at the 2, 5, and 10 min periods, whereas the PS of the [Formula: see text], [Formula: see text], and [Formula: see text] responses was not significantly different between normoxia and hypoxia at any period. These findings suggest that the lesser changes in Amp and PS in ventilatory and gas exchange kinetics during walking at a sinusoidally changing speed were remarkably different from a deceleration in HR kinetics under moderate hypoxia.