Older adults amplify passive gait stability during obstacle crossing without weakening the stabilizing synergy

Older adults sustain disproportionately severe injuries from trip-induced falls during obstacle crossing. Such falls depend partly on forward momentum when the foot crosses the obstacle. MOS _AP , an index of passive dynamic gait stability, reflects this momentum. We quantified MOS _AP and a synergy index from uncontrolled manifold analysis of step length and extrapolated center of mass in 25 young (21.6 ± 3.5 yr) and 23 older adults (68 ± 4.3 yr) during unobstructed and obstructed walking, to test whether MOS _AP increases during obstacle crossing and whether it is actively stabilized at each step. Both groups increased MOS _AP progressively over two approach steps by reducing forward momentum and shifting the center of mass posteriorly. Older adults showed greater increases at the crossing steps. The synergy index was positive for all steps, showing that deviations in step length and extrapolated center of mass covaried to stabilize MOS _AP at step-specific values. The synergy index was not influenced by age. We conclude that adults actively recruit passive body mechanics while approaching and crossing obstacles to reduce the risk of a trip becoming a fall. Older adults amplify this strategy to compensate for diminished neuromuscular corrective capabilities.