Foot placement control impairments in persons with chronic stroke become more evident during optic-flow perturbations

Mediolateral balance during gait relies on regulation of the body’s centre of mass (CoM) through accurate foot placement. Despite weaker CoM-foot placement coupling in persons with chronic stroke (PwCS), they often maintain dynamic balance under steady-state conditions. In daily life, unexpected perturbations often require greater reliance on reactive balance control, which may be particularly challenging for PwCS but remains underexplored. This study examined the effect of destabilizing optic-flow perturbations on step-to-step foot placement control in PwCS compared with controls, relative to steady-state gait. Twenty PwCS and 16 controls walked on an instrumented treadmill in a virtual reality environment under three conditions: unperturbed gait and continuous mediolateral optic-flow perturbations of moderate and strong intensity. Perturbation effects on CoM dynamics were quantified using the excursion and variability of the extrapolated CoM (xCoM). Step-to-step foot placement control was assessed as the RMSE between actual and model-predicted foot placements from a linear regression using CoM position and velocity. Mean and variability of the margin of stability were also calculated. Group (paretic, non-paretic, control), condition, and their interaction were evaluated using linear mixed effects models, and exploratory correlations of foot placement deviation across conditions were examined. Optic-flow perturbations increased xCoM variability (p < 0.001), with no group × condition interaction (p ≥ 0.08). Foot placement deviation (RMSE) was higher in PwCS across all conditions (all p < 0.01) and, compared to controls, showed a significant group × condition interaction at strong intensity (paretic and non-paretic both p = 0.01), but not at moderate intensity (both p = 0.07). Foot placement control did not differ between paretic and non-paretic legs (p ≥ 0.60). PwCS showed larger average margins of stability and greater increases in variability with perturbation intensity than controls (all p ≤ 0.01). Correlations between unperturbed and perturbed conditions were weak to moderate (r = 0.2–0.45) but strong between perturbation intensities (r ≥ 0.68). PwCS exhibited impaired mediolateral foot placement control, which became more pronounced when reactive balance demands increased. Challenging balance through continuous optic-flow perturbations exacerbates impairments in step-to-step balance control after stroke.