Stepping behaviour contributes little to balance control against continuous mediolateral trunk perturbations.

Human bipedal gait is exceptionally stable, but the underlying strategies to maintain stability are unclear, especially in the frontal plane. Our study investigated balance strategies of healthy adults subjected to continuous mediolateral oscillations at the trunk during walking. We used a backpack with a passive inverted pendulum to create perturbations that were fixed, in-phase or out-of-phase with subjects' trunk. We evaluated subjects' corrective strategies and whether they yielded equivalent stability, measured by the margin of stability and the local divergence exponent. The margin of stability measure quantified adjustments in step behaviour relative to the centre of mass, and the local divergence exponent measure characterized the chaotic behaviour of the system throughout the entire trial. Among the conditions, there was no significant difference in the step width. We found a higher margin of stability for the out-of-phase condition and the lowest local divergence exponent for the in-phase condition and the highest for the fixed condition. These results indicate that the in-phase condition was more stable with respect to fluctuations throughout gait cycles, and the out-of-phase condition was more stable in terms of foot placement relative to centre of mass. To maintain equivalent or greater gait stability, subjects elected to reduce the motion of their centre of mass rather than alter step width. The reduction in centre of mass motion without a reduction in step width suggests direct control of the centre of mass to maintain stability was preferred over adjusting stepping behaviour.