Aging limits the reduction of reactive muscle activity to perturbations but spares the retrieval of context-specific patterns with practice

Moving smoothly through a changing environment requires immediate reactive responses to maintain balance at an environmental transition, and learned, context-specific responses to efficiently meet the new environmental demands. People use both the reactive and context-specific responses to adapt to new environment and to re-adapt more rapidly on subsequent re-encounters. While re-adaptation slows with older age, the underlying muscular mechanisms are unknown. We hypothesized that practice would reduce reactive recruitment and increase context-specific recruitment and that both changes would be smaller in older adults. To test this, younger and older adults practiced split-belt walking (one belt faster than the other) over three days. We recorded surface electromyography from 10 muscles per leg and used linear regression to decompose each session's activity into a reactive component (template from naive, early exposure) and a context-specific component (template from practiced activity). Mixed-effects models showed that, with practice, older adults decreased reactive recruitment less than younger adults, particularly in distal muscles, with no strong evidence for an age difference in context-specific recruitment. In sum, with practice, aging limits the reduction of reactive responses to a novel walking context but spares the retrieval of context-specific muscle patterns, suggesting that mobility rehabilitation for older adults could target reactive mechanisms.