Humans choose trunk lean angles consistent with minimal metabolic costs—an integrative experimental and musculoskeletal simulation approach

Despite the substantial mass and moment of inertia of the trunk, its influence on minimizing metabolic cost in human locomotion is poorly understood. We show that self-selected trunk leaning in humans aligns with minimized metabolic costs during running. Utilizing an integrative experimental and musculoskeletal simulation approach, we reveal how trunk leaning redistributes joint moments and metabolic demands. We report estimates of muscle-specific changes in metabolic demands and contraction conditions for key muscles, including the hamstrings, uniarticular hip extensors and plantar flexors, reflecting the complex interplay between kinetic demands and muscle energetics. Our results show that leaning forward reduces knee extension moments but involves metabolic cost trade-offs. Our findings indicate that the observed U-shaped whole-body metabolic cost curve is attributed to intricate and offsetting changes in metabolic cost among multiple muscle groups across trunk leaning conditions, reflecting the fundamental impact of trunk kinematics on running biomechanics.