Stepping in the direction of the fall: the next foot placement can be predicted from current upper body state in steady-state walking

Abstract

During human walking, perturbations to the upper body can be partly corrected by placing the foot appropriately on the next step. Here, we infer aspects of such foot placement dynamics using step-to-step variability over hundreds of steps of steady-state walking data. In particular, we infer dependence of the 'next' foot position on upper body state at different phases during the 'current' step. We show that a linear function of the hip position and velocity state (approximating the body center of mass state) during mid-stance explains over 80% of the next lateral foot position variance, consistent with (but not proving) lateral stabilization using foot placement. This linear function implies that a rightward pelvic deviation during a left stance results in a larger step width and smaller step length than average on the next foot placement. The absolute position on the treadmill does not add significant information about the next foot relative to current stance foot over that already available in the pelvis position and velocity. Such walking dynamics inference with steady-state data may allow diagnostics of stability and inform biomimetic exoskeleton or robot design.

Center of mass velocity based predictions in balance recovery following pelvis perturbations during human walking

“…This is in line with previous findings in ML foot placement (Hof et al, 2007;Wang and Srinivasan, 2014). Findings in unperturbed walking have suggested that the AP COM velocity at mid-stance also significantly contributes to predictions of the next AP foot placement location (Wang and Srinivasan, 2014).…”

“…This location was expressed relative to the trailing foot, and therefore contained effects occurring between the COM and both the leading and the trailing foot. Our results suggest that the findings for AP foot placement in Wang and Srinivasan (2014) are mainly caused by changes between the COM and the trailing foot. Here, none of the AP perturbations led to a distance between the COM and the leading foot that was significantly different from the distance in unperturbed walking.…”

“…The horizontal position and velocity of the COM have predictive properties in human foot placement. Using the pelvis as an approximation of the COM, a linear function of the ML pelvis position and velocity relative to the stance foot at mid-stance could be used to predict over 80% of the variance in ML foot placement during unperturbed human treadmill walking (Wang and Srinivasan, 2014). Pelvis predictive power was lower for AP foot placement, explaining just over 30% at midstance.…”

Walking on a moving surface: energy-optimal walking motions on a shaky bridge and a shaking treadmill can reduce energy costs below normal

“…From a mechanical perspective, the most significant and plausible complement to the energy optimality hypothesis is the hypothesis that the observed human-platform dynamics is an emergent property of the controller that keeps human walking stable. While limited theoretical explorations [7,36] have found evidence for the stability hypothesis, a definitive understanding of the human-platform dynamics has to await a more detailed characterization of the controller that maintains stability during walking-an outstanding open problem in locomotion biomechanics [10,11,47]. Once we have this human walking controller, we could check if one or many biped models endowed with such a stabilizing controller, walking forward on a shaky bridge, automatically synchronize with each other and to the bridge, thereby causing large bridge oscillations.…”

“…not fall down. Wider step widths might improve sideways stability [9] and humans use sideways foot-placement to avoid falling sideways and recover from sideways perturbations [10,11]. Indeed, a few authors [7,8] have used 'inverted pendulum models' of lateral pedestrian motion with step-width-control-based stabilization to explain pedestriandriven bridge oscillations.…”