Body-foot geometries as revealed by perturbed obstacle position with different time constraints.

This study examined the geometrical relationships between the feet, pelvis and an environmental obstruction when crossing an obstacle with unexpected changes to its position. Nine healthy young adults stepped over an obstacle 19 cm high with their right leg leading. The obstacle could be static or advanced at either lead (early detection) or trail (late detection) foot contact prior to clearance to force an adaptive reorganization of body-foot geometry and foot proximity to the obstacle. Stride length, minimum foot clearance over the obstacle, and foot-obstacle horizontal proximity before and after clearance were measured along with the relative position of the pelvis to each foot at eight points (four for each foot) during approach and clearance: heel contacts before and after crossing the obstacle, maximum foot heights and foot clearances. With early obstacle movement, trail limb stride length before crossing was lengthened, but foot proximity was still far from the final obstacle position. Clearance was less affected for the trail foot as compared to the lead foot. Proximity of the lead limb following clearance was the same for both early and late perturbations and closer than for the static obstacle condition. For relative body-foot positioning, significant differences were found only in the anterior-posterior direction. Following obstacle displacement, body-foot geometry was initially adapted, but then re-established to static obstacle values with an apparent focus on a balance geometry with the forward placed foot establishing new contact. These findings support an overall balance geometry that can be temporarily adjusted and coordinated with foot proximity to the obstruction to maintain continual gait and safe clearance.