The energetic cost of human standing balance and gait initiation over a range of natural postures.

Human movement control is shaped by competing objectives, among which minimizing energy expenditure plays a central role, particularly in determining preferred walking patterns. Whether energetic cost similarly influences standing balance remains unclear because it has not been systematically quantified across a range of natural postures. Importantly, standing is the resting state from which most walking begins, suggesting that the optimization of posture may also reflect the energetic demands of initiating gait. In this study, we use a combination of indirect calorimetry and musculoskeletal simulations to characterize the energetic cost of standing and gait initiation across natural standing postures and investigate whether humans optimize energy expenditure under these conditions. In Experiment 1 (N = 13), we measured metabolic cost at preferred and six different prescribed whole-body lean angles. Energy expenditure was lowest at a slight anterior lean (1.15°) and increased monotonically with whole-body lean angle in either direction, rising twice as fast posteriorly compared to anteriorly. This asymmetry challenges the common modeling simplification that effort is symmetric and linear or quadratic with lean angle. Furthermore, participants preferred body angles (1.50 ± 0.73°) with similar energy expenditure to the minimum-cost lean but with significantly more postural variability, suggesting that strict postural regulation was not necessary for minimizing energetic cost. In Experiment 2 (N = 20), participants initiated forward and backward walking from preferred or prescribed lean angles. Participants did not alter their standing posture before expected gait initiations in the forward or backward direction, consistent with musculoskeletal simulations showing that leaning further in the anticipated direction did not significantly improve gait initiation time or energetic costs. Together, these findings suggest that postural strategies optimize energy efficiency when permitted by the demands of movement readiness. Our study quantifies the energetic cost landscape that governs human postural control, challenges widely used symmetric estimations of this cost, and offers an empirical foundation for developing more accurate simulations of posture and energy expenditure.