The Sequence (S) index as a marker of diminished step-to-step transition efficiency in older adults.

Introduction: Age-related declines in walking efficiency are often attributed to musculoskeletal and neuromuscular changes, yet the mechanisms underlying these inefficiencies remain poorly understood. One key contributor is the step-to-step transition (STST), during which the center of mass (CoM) is redirected between limbs. The Sequence (S) index quantifies the temporal overlap between push-off and collision forces during STST, with higher values indicating greater mechanical energy loss. This study investigated whether aging affects STST efficiency, as quantified by the S index, and examined the timing and impulse characteristics underlying observed differences. Methods: Thirteen young and eleven older healthy adults walked at five speeds relative to their preferred walking speed on an instrumented treadmill while ground reaction force data were collected. Results: Older adults exhibited significantly higher S index values across all speeds, indicating less efficient gait. These differences were accompanied by shorter pre-HC duration, lower total push-off impulse, lower single support push-off impulse, and higher double support collision impulse. Both groups showed reductions in S index at higher speeds, primarily through increased single-support timing and impulses, and decreased double-support overlap, but older adults remained less efficient overall. Discussion: These results suggest that aging impairs the temporal and mechanical coordination of STST. This effect may potentially be due to neuromuscular changes. The S index offers a step-level, mechanically grounded metric for assessing gait efficiency and may provide insight into energetic cost in older populations.