How older adults maintain lateral balance while walking on narrowing paths

Background: Older adults have difficulty maintaining side-to-side balance while navigating daily environments. Losing balance in such circumstances can lead to falls. We need to better understand how older adults adapt lateral balance to navigate environment-imposed task constraints. Research Question: How do older adults adjust mediolateral balance while walking along continually-narrowing paths, and what are the stability implications of these adjustments? Methods: Eighteen older (71.6±6.0 years) and twenty younger (21.7±2.6 years) healthy adults traversed 25 m-long paths that gradually narrowed from 45 cm to 5 cm. Participants switched onto an adjacent path when they chose. We quantified participants’ lateral center-of-mass dynamics and lateral Margins of Stability (MoS_L) as paths narrowed. We quantified lateral Probability of Instability (PoI_L) as the probability that participants would take a laterally unstable (MoS_L<0) step as they walked. We also extracted these outcomes where participants switched paths. Results: As paths narrowed, all participants exhibited progressively smaller average MoS_L and increasingly larger PoI_L. However, their MoS_L variability was largest at both the narrowest and widest path sections. Older adults exhibited consistently both larger average and more variable MoS_L across path widths. Taken into account together, these resulted in either comparable or somewhat larger PoI_L as paths narrowed. Older adults left the narrowing paths sooner, on average, than younger. As they did so, older adults exhibited significantly larger average and more variable MoS_L, but somewhat smaller PoI_L than younger. Significance: Our results directly challenge the predominant interpretation that larger average MoS_L indicate “greater stability”, which we argue is inconsistent with the principles underlying its derivation. In contrast, analyzing step-to-step gait dynamics, together with estimating PoI_L allows one to properly quantify instability risk. Furthermore, the adaptive strategies uncovered using these methods suggest potential targets for future interventions to reduce falls in older adults.