Reduced center of pressure modulation elicits foot placement adjustments, but no additional trunk motion during anteroposterior-perturbed walking.

Understanding balance during human gait is complicated by the abundance of recovery options. Among all possible recovery options, three main strategies are often considered for human balance control, being the ankle, hip, and foot placement strategies. All can be addressed when balance is threatened during walking, but their relative importance remains uncertain. We have previously shown that healthy human subjects did not significantly adjust their foot placement relative to the body's center of mass (COM) in the first recovery step following anteroposterior pelvis perturbations, as compared to unperturbed walking. An ankle strategy could have contributed to the recovery instead. Here the goal is to further elucidate balance strategy preferences by investigating the stepping and hip strategies following these anteroposterior perturbations, but with an ankle strategy made ineffective. This was achieved by physically blocking each ankle and minimizing the support area of each foot through a pair of modified ankle-foot orthoses. These "pin-shoes" enabled stilt-like walking and ensured that foot placement adjustment was the only way to modulate the center of pressure location, comparable to "footless" inverted pendulum models of walking. Despite the pin-shoes, subjects did not additionally address a hip strategy compared to normal walking, but relied on foot placement adjustments instead. The observed foot placement adjustments were furthermore in line with concepts derived from a linear inverted pendulum model of walking. These results suggest low hip strategy priority when a foot placement strategy is available, while the latter can be predicted with concepts derived from a simple walking model.