Abstract
We investigated the effects of moderate hypoxia (FiO2 = 15%) on different kinetics between pulmonary ventilation () and heart rate (HR) during treadmill walking. Breath-by-breath , oxygen uptake (), carbon dioxide output (), 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 and ) and hyperventilation. The Amp values of the , , and 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 , , and 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.
Highlights
Hiking is a very popular pastime in Japan—approx. "300,000 people have hiked to the top of Mount Fuji alone
The slopes of the V_E and V_CO2 relationship were not different between normoxia and hypoxia, which suggests that the ventilatory adjustment of hormonal drive was equivalent between normoxia and hypoxia, even though hypoxic-induced hyperventilation occurred via the peripheral chemoreflex during sinusoidal walking
Moderate hypoxia could achieve an increase in the energy expenditure and the increased V_E appeared to have occurred via the peripheral chemoreflex during non-steady-state walking at a sinusoidally changing speed
Summary
Hiking is a very popular pastime in Japan—approx. "300,000 people have hiked to the top of Mount Fuji alone. Modeling the experience of mountain hiking in an experimental setting can be difficult, since mountain landscapes are rugged and the atmosphere is often hypoxic. In an attempt to accurately model the physiological cardiovascular responses of walking along mountain paths, we used treadmill walking in combination with the sinusoidal oscillation of walking speed and changes in the inspiratory oxygen concentrations (FiO2). Ventilation and heart rate kinetics during walking under hypoxia. Dynamic physiological responsiveness can be estimated by the phase shift (PS) as the time lag and the amplitude (Amp) of the heart rate (HR) ventilatory and gas exchange variables in response to sinusoidal exercise [3,4,5,6], which can provide a clear and simple evaluation of an individual’s HR and ventilatory and gas exchange responses
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