Different stabilizing mechanisms but a common task-level aim in standing and walking

Abstract

Intrinsic mechanisms and feedback control act together to stabilize the body in upright tasks. In unperturbed standing and walking, their combined effects can be captured by lumped stabilization models, relating delayed center of mass position and velocity information to current ground reaction forces. If and how lumped stabilization parameters change across these tasks is unclear. We applied a stabilization model to estimate and compare stabilization between unperturbed standing and walking. Fifteen healthy young participants (21+/- 4 yrs, 63 +/- 9 kg, 1.70 +/-0.10 m, 13 females, 2 males) walked at 1.25 m/s for 5 minutes, and performed 3 different standing tasks: normal standing, unipedal standing, and step posture for 1 minute, repeated 5 times. Whole-body kinematics and ground reaction forces were collected and used to fit the stabilization model and estimate the effective delay and lumped gains. Only small differences were found between standing tasks. Model fits were significantly higher in standing than in walking. ML effective delays were significantly longer in walking than in standing, whereas AP delays were comparable. The stabilization gains varied significantly across tasks and directions. The lumped position gains in most tasks exceeded critical stiffness, except for the mean values in walking, which were lower than the critical stiffness. Lumped velocity gains were all at under-damped level. The ratio of lumped position to velocity gains in standing were consistently close to human body's eigenfrequency (sqrt(g/l)) as predicted by the extrapolated center of mass concept. In walking, the ratio was close to the eigenfrequency during phases in which stabilization was significant. Our findings suggest that stabilization is organized at the task level, with lumped effects of all stabilizing mechanisms acting to preserve a consistent weighting between position and velocity contributions across tasks, effectively regulating the CoM motion to follow a pendulum-like trajectory.