ORIGINAL RESEARCH article
Front. Hum. Neurosci.
Sec. Motor Neuroscience
This article is part of the Research TopicWomen in motor neuroscience 2025View all 7 articles
The Sequence (S) Index as a Marker of Diminished Step-to-step Transition Efficiency in Older Adults
Provisionally accepted
- 1Macon & Joan Brock Virginia Health Sciences at Old Dominion University Ellmer College of Health Sciences, Norfolk, United States
- 2University of Wisconsin-Madison College of Engineering, Madison, United States
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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.
Keywords: Aging, energy loss, Gait efficiency, Ground reaction forces, S index, step-to-step transition
Received: 22 Sep 2025; Accepted: 30 Jan 2026.
Copyright: © 2026 Alijanpour, Kulkarni, Adamczyk and Russell. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Elham Alijanpour
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