Step length synergy is weaker in older adults during obstacle crossing.
Community ambulation requires gait adaptations to navigate environmental obstacles. It is well known that while crossing obstacles, variables quantifying the gait pattern are controlled relative to the obstacle's position. However, the stability of these gait variables is underexplored. We measured foot positions relative to an obstacle as young and older adults stepped over it. We report secondary analysis of this data in which we quantified the stability of the step length when the two feet are placed on either side of the obstacle. We employed the uncontrolled manifold approach to test the hypotheses that (1) synergistic across-trial co-variation in the distances of the front and the back heel from the obstacle edge will stabilize the step length, and (2) older adults will display weaker synergies (i.e., lower step length stability). We observed that the front and back heel distances relative to the obstacle's edge co-varied synergistically to stabilize the step length for both age groups. Therefore, foot placement during obstacle navigation is controlled not only with reference to a feature of the environment (i.e. the obstacle), but also to stabilize the step length, presumably to control COM motion. The synergy index was 38% lower for older adults than young adults. This decline may be associated with aging-related functional deficits and tripping-related falls.