The dual timescales of gait adaptation: Initial stability adjustments followed by subsequent energetic cost adjustments.

Gait adaptation during bipedal walking allows people to adjust their walking patterns to maintain balance, avoid obstacles, and avoid injury. Adaptation involves complex processes that function to maintain stability and reduce energy expenditure. However, the processes that influence walking patterns during different points in the adaptation period remain to be investigated. We recruited seventeen young adults ages 19-35 to assess split-belt adaptation. We also assessed individual aerobic capacity to understand how aerobic capacity influences adaptation. We analyzed step lengths, step length asymmetry (SLA), mediolateral margins of stability, positive, negative, and net mechanical work rates, as well as metabolic rate during adaptation. We used dual-rate exponential mixed-effects regressions to estimate the adaptation of each measure over two timescales. Our results indicate that mediolateral stability adapts over a single timescale in under 1 minute, while mechanical work rates, metabolic rate, step lengths, and step length asymmetry adapt over two distinct timescales, ranging from 3.5 to 11.2 minutes. We then regressed mediolateral margins of stability, net mechanical work rate, and metabolic rate on step length asymmetry during early and late adaptation phases to determine if stability drives early adaptation and energetic cost drives late adaptation. Stability predicted SLA during the initial rapid onset of adaptation, and mechanical work rate predicted SLA during the latter part of adaptation. These findings suggest that stability optimization may contribute to early gait changes and that mechanical work contributes to later changes during adaptation. A final sub-analysis assessed the effect of aerobic capacity on step length asymmetry adaptation. Aerobic capacity levels below 36 and above 43 ml/kg/min resulted in greater adaptation, underscoring the metabolic influences on gait adaptation. This study illuminates the complex interplay between biomechanical and metabolic factors in gait adaptation, shedding light on fundamental mechanisms underlying human locomotion.