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

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.

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.

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.

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.