Ground reaction forces intersect above the center of mass even when walking down visible and camouflaged curbs

A main objective in bipedal walking is controlling the whole body to stay upright. One strategy that promotes this objective is to direct the ground reaction forces (GRFs) to a point above the center of mass (COM). In humans, such force patterns can be observed for unperturbed walking, but it is not known whether the same strategy is used for a walkway that changes in height. In this study, 11 volunteers stepped down off a visible (0, 10 and 20 cm) and a camouflaged (0 or 10 cm) curb while walking at two different speeds (1.2±0.1 and 1.7±0.1 m s^-1). The results showed that in all conditions the GRFs pointed predominantly above the COM. Vectors directed from the center of pressure (COP) to the intersection point (IP) closely fitted the measured GRF direction not only in visible conditions (R²>97.5%) but also in camouflaged curb negotiation (R²>89.8%). Additional analysis of variables included in the calculation of the IP location showed considerable differences for the camouflaged curb negotiation: compared with level walking, the COP shifted posterior relative to the COM and the vertical GRFs were higher in the beginning and lower in later parts of the stance phase of the perturbed contact. The results suggest that IP behavior can be observed for both visible and camouflaged curb negotiation. For further regulation of the whole-body angle, the asymmetrical vertical GRFs could counteract the effect of a posterior shifted step.