Characterization of the effects of sex, speed, and incline on metabolic rate during partial gravity ambulation

With the Artemis missions returning humans to the Moon, characterizing ambulation and metabolic rate in partial gravity environments is critical for spacesuit design, mission planning, and architecture. To date, no study has assessed the combined effects of sex, speed, and incline on metabolic rate across a broad range of partial gravity levels. In this study, 12 subjects ambulated on a treadmill under 16 conditions combining speeds (3 mph and 4 mph), gravity levels (0.25 g, 0.5 g, 0.75 g, and 1 g), and inclines (0% and 10%). A novel body weight support system, the Portable Offloading for Walking, Exercise, and Running (POWER) device, was used to simulate partial gravity. Metabolic rate was measured with a COSMED K5 system. Dose-response curves were constructed using a generalized linear mixed-effects model, revealing significant increases in metabolic rate with speed (<i>P</i> < 0.001), gravity level (<i>P</i> < 0.001), incline (<i>P</i> < 0.001), and significant interactions between speed and gravity level (<i>P</i> = 0.022) and between gravity level and incline (<i>P</i> < 0.001). Each partial gravity level differed significantly from the 1-g baseline in all ambulation conditions. Sex differences were statistically significant only at 4 mph, 10% incline, and 1 g (<i>P</i> = 0.002), the most challenging condition. These results advance understanding of energy expenditure during ambulation in reduced gravity and provide valuable data to support astronaut training and spacesuit development for future planetary exploration.<b>NEW & NOTEWORTHY</b> We characterized the effects of sex, speed, and incline on metabolic rate in 12 subjects (6 females) across partial gravity levels (0.25-1 g). Model results showed significant increases in metabolic rate with speed, gravity, incline, and interactions between speed and gravity, and gravity and incline. Sex differences appeared only in the most demanding condition (4 mph, 10% incline, 1 g). This framework quantifies key variables of partial gravity ambulation relevant to planetary exploration.