Abstract
Walking humans respond to pulls or pushes on their upper body by changing where they place their foot on the next step. Usually, they place their foot further along the direction of the upper body perturbation. Here, we examine how this foot placement response is affected by the average step width during walking. We performed experiments with humans walking on a treadmill, both normally and at five different prescribed step widths. We prescribed step widths by requiring subjects to step on lines drawn on the treadmill belt. We inferred a linear model between the torso marker state at mid-stance and the next foot position. The coefficients in this linear model (which are analogous to feedback gains for foot placement) changed with increasing step width as follows. The sideways foot placement response to a given sideways torso deviation decreased. The fore–aft foot placement response to a given fore–aft torso deviation also decreased. Coupling between fore–aft foot placement and sideways torso deviations increased. These changes in foot placement feedback gains did not significantly affect walking stability as quantified by Floquet multipliers (which estimate how quickly the system corrects a small perturbation), despite increasing foot placement variance and upper body motion variance (kinematic variability).
Highlights
Humans modulate their foot placement during active stabilization of walking [1,2,3]
We find that the coefficients in this linear model—the effective ‘foot placement control gains’—change systematically with step width
‘Cautious walking’ is sometimes described as having larger step widths and slower speeds [12]. And those prone to falling walk with larger step widths, perhaps to compensate for other factors that decrease their walking stability [22,26]. (They often have increased step-width variability [27].) this possibly increased stability from having a wide base of support may come at the cost of higher energy cost [28]
Summary
Humans modulate their foot placement during active stabilization of walking [1,2,3]. Foot placement allows humans to change their leg direction, thereby changing their leg force direction during2017 The Authors. 2 To recover from this perturbation, they may place their right foot further to the right than normal, so that the right leg force has a greater leftward component, acting as a restoring force [2,4]. Such foot placement reactions have been found in response to perturbations to the upper body [3,5] and have been inferred from steady-state walking data [6,7,8]. Persistent pseudorandom visual and mechanical perturbations increase step length and step-width variability, suggesting (but not demonstrating) the use of foot placement in response to the perturbations [9,10,11]
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