Human metabolic heat production under varying atmospheric and oxygen pressures during submaximal cycling.

Accurate thermal management in hypoxic and altered-pressure environments is essential, as variations in ambient pressure and oxygen availability can modify human metabolic heat production. This study investigated human metabolic responses under seven atmospheric conditions (normobaric, hypobaric, and micro-hyperbaric) with varying oxygen concentrations. Nine healthy male participants performed submaximal cycling while minute ventilation, respiration rate, respiratory quotient, body temperature, heart rate, and blood lactate were measured, along with subjective thermal sensation and fatigue. Metabolic rate was calculated from gas exchange. Resting metabolic rate significantly decreased under hypobaric and micro-hyperbaric conditions and partially recovered with oxygen supplementation. During exercise, similar trends were observed, except for a significant metabolic increase at 40 kPa micro-hyperbaric pressure. Blood lactate increased in hypobaric environments but decreased under micro-hyperbaric ones. Overall, metabolic heat production depends on both atmospheric pressure and oxygen partial pressure, suggesting that oxygen availability alone cannot fully explain thermophysiological responses in extreme environments.