The relationship between ankle-foot work and walking stability across age and walking slope

Bipedal walking requires frontal plane balance control and forward propulsion on a step-by-step basis. Understanding how changes in ankle-foot push-off work influence walking stability is a crucial first step toward identifying whether enhancing ankle-foot positive work in turn enhances or challenges balance control-particularly in older adults and when walking on slopes where push-off work and stability demands change. A total of 20 younger adults (10F / 10 M) and 20 older adults (10F / 10 M) walked barefoot on a split-belt instrumented treadmill for six minutes at fixed speed (1.2 m/s) in three different slope conditions: level (0°), incline (5°), and decline (5°). Positive ankle-foot work (W_ankle-foot⁺) during stance, walking stability quantified using the medial-lateral margin of stability (MoS_ml), mediolateral center of mass velocity (V_ml), and step width (SW) were calculated on a step-to-step basis. Separate linear mixed-effects regression models tested the association between the step-to-step change in each variable, relative to the mean of the overall trial, ΔMoS_ml, ΔV_ml, and ΔSW and the independent predictor variable of ΔW_ankle-foot⁺. We observed a negative association between ΔW_ankle-foot⁺ and ΔMoS_ml (all p < 0.001) during each walking slope condition for both younger and older adults. The relationship between foot-ankle positive work and stability appeared to be driven by changes in ΔV_ml (all p < 0.001). Our results empirically demonstrate the inverse association between ΔW_ankle-foot⁺ and ΔMoS_ml on a step-by-step basis (within walking slope condition).