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

Abstract

Humans typically select movements that minimize energetic cost, a principle most clearly observed during locomotion. Whether such optimization of energy expenditure also governs standing balance remains unclear because its energetic cost has not been systematically quantified across a range of natural postures. Moreover, because standing is often the resting state from which most walking begins, 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 orientations. Energy expenditure was lowest at a slight anterior orientation (1.15{degrees}) and increased monotonically with whole-body angle, 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 orientations (1.50 {+/-} 0.73{degrees}) with similar energy expenditure to the minimum-cost orientation but with significantly more postural variability, suggesting that strict postural regulation was not necessary for energy-optimal control. In Experiment 2 (N = 20), participants initiated forward and backward walking from preferred or prescribed lean orientations. 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 inverted pendulum estimations of this cost, and offers an empirical foundation for developing more accurate simulations of posture and energy expenditure. Author summaryHumans are thought to move in ways that save energy. This idea is well supported for walking, but it is not known whether we do the same during quiet standing. Furthermore, because standing is our idle state from which we initiate movement, we may optimize our posture to ease these transitions. In this study, we investigated whether humans stand in postures that minimize energy expenditure. First, we measured and simulated how the cost of posture changes over a range of natural whole-body orientations and determined that humans tend to choose postures close to the orientation with the lowest cost. Leaning backward incurs an additional energetic cost at twice the rate of forward-leaning postures. Second, we investigated whether expecting to walk in the forward or backward direction affects our preferred posture. Surprisingly, participants did not change their posture in preparation for the known direction of walking. Simulations demonstrated that the energetic benefit of doing so was small. Overall, our findings show that maintaining a slight forward lean results in optimal energy expenditure during standing and gait initiation. However, commonly used assumptions of how energy expenditure varies with lean angle do not match the measured cost distribution and those predicted by musculoskeletal simulation.