The effect of optical flow perturbations on walking foot placement control in people with multiple sclerosis

People with multiple sclerosis (PwMS) experience elevated fall risk, yet traditional gait measures such as step variability provide limited insight into underlying control mechanisms. We examined step-to-step regulation of foot placement in PwMS versus age-matched controls during treadmill walking with and without anteroposterior (AP) and mediolateral (ML) optical flow perturbations. Fifteen PwMS and fifteen matched controls completed three-minute walking trials while kinematics were recorded. Detrended fluctuation analysis (DFA) assessed the temporal organization of step width (SW) and step length (SL) time series, and direct control analysis (DCA) quantified stride-to-stride corrections. DFA showed lesser SW persistence during ML perturbations compared to other conditions, indicating tighter regulation under lateral balance challenges. A group × condition interaction revealed that PwMS, but not controls, exhibited reduced SL persistence during both AP and ML perturbations. These results highlight broader disruptions in temporal organization of stepping in PwMS. DCA revealed that both groups increased corrective responses to SW deviations under ML perturbations, but model fit (R²) values were lower, indicating reduced consistency than unperturbed walking. Additionally, PwMS showed reduced SW corrections during AP perturbations. In contrast, neither SL regression coefficients nor R² values differed by group or condition. Together, DFA and DCA suggest that PwMS exhibit disrupted temporal structure of foot placement regulation, particularly for SL under visually destabilizing conditions, and that stronger but less consistent SW corrections emerge under ML challenges. These complementary analyses provide novel insight into mechanisms of walking instability in PwMS and may inform interventions targeting reliable step-to-step control.