Mechanical and metabolic consequences of sagittal trunk lean angle in walking – a dynamic walking perspective

Bipedal walking requires a balance of muscle work and energy losses, with models indicating that powering gait with ankle push-off is more energetically economical than powering with hip joint work. This study investigates how varying trunk lean angle affects joint mechanics and energy expenditure during walking. We hypothesized that leaning forward would increase hip work, reduce ankle push-off and increase energy consumption, and leaning backward would have complementary effects. Healthy young adults walked at 1.3 m s-1 while adjusting their trunk angles from backward 15 deg to forward 60 deg using visual feedback from a chest-mounted inertial motion sensor. Center of mass (COM) mechanics and lower-body joint mechanics were estimated using motion capture and force treadmill measurements, alongside metabolic rate using respirometry. With forward trunk lean, COM work became more negative during collisions, increased in the middle of stance phase and was reduced in push-off. At the joint level, forward trunk lean led to increasing stance-phase hip moment and hip work, while ankle work decreased for moderate trunk angles. The early vertical ground reaction force peak and loading rate also increased with forward trunk lean. Backward trunk lean led to reduced hip work, increased ankle work and increased push-off work. Metabolic rate was minimized in the 0 deg condition and increased with trunk lean in either direction. Trunk lean significantly impacts lower-limb mechanics and energy consumption, with a trade-off between hip and ankle work, suggesting potential applications for improving walking in populations with diminished push-off, such as older adults.