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

Abstract

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 (R 2 >97.5%) but also in camouflaged curb negotiation (R 2 >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.

Postural stability in human running with step-down perturbations: an experimental and numerical study

“…Thus, we can directly compare the R 2 values for both walking and running. The R 2 value of the level running is 6 percentage points lower than the R 2 reported in [5] for level walking. The R 2 value for V10 running is 15 percentage points lower than V10 walking, whereas the R 2 for C10 running is up to 25 percentage points lower compared to C10 walking.…”

“…Among those, only a single study reports a limited set of level walking trials with a VP below the CoM (VP B ) [1]. The VP A strategy is also observed when coping with the step-down perturbations in human walking, even when walking down a camouflaged curb [5]. A similar behaviour is observed for the avians, where a VP A of 5 cm is reported for level walking, grounded running, and running of the quail [6,7].…”

“…The method for analysing the gait data and estimating a potential VP is analogous to the gait analysis carried out for the human walking in [5]. Here, we denote the intersection point of the GRF vectors as a VP without implications for this point being above or below the CoM.…”

“…Here, we consistently remove the first 10% of the contact phase. Our experimental set-up and methodology are identical to [5], which reports results from human walking. Thus, we can directly compare the R 2 values for both walking and running.…”

“…One major observation states that the ground reaction forces (GRFs) intersect near a virtual point (VP) above the centre of mass (CoM) [1]. Subsequent gait studies report that the VP is 15-50 cm above the CoM (VP A ) in the sagittal plane for level walking [1][2][3][4][5]. Among those, only a single study reports a limited set of level walking trials with a VP below the CoM (VP B ) [1].…”