The effects of gait speed on the responses to immediate and prolonged exposure to mediolateral optic flow perturbation in healthy young adults.

Optic flow is vital for locomotor control and is often perturbed to study the impact of optic flow on balance control. However, it remains unclear whether gait speed influences responses to such perturbations. This study aims to examine the effects of gait speed on gait parameters following immediate and prolonged exposure to mediolateral optic flow perturbations. Twenty-one young adults (23.43 ± 4.19 years) walked on an instrumented treadmill, including 3 phases: baseline (3 min), perturbation with mediolateral optic flow (8 min), and post-perturbation (3 min). Trials were conducted at 0.6, 1.2, and 1.8 m/s. Ground reaction forces and 3D motion data were collected to calculate mediolateral margin of stability (MoS), mean step length (SL), step width (SW) and their variabilities. Three repeated-measures ANOVAs (Speed by Phase) were used to compare: baseline vs. early perturbation, early vs. late perturbation, and baseline vs. post-perturbation. The responses to immediate and prolonged exposure to optic flow perturbation were speed dependent. Walking at slow speeds induced greater immediate responses in mediolateral gait parameters (SW and mediolateral MoS, both p < 0.001) compared to walking at faster speeds. During the perturbation phase, the adaptations were larger at faster vs. slower speeds for gait parameters in the direction of movement (SL, p = 0.007). Immediate responses and adaptations to mediolateral optic flow perturbations are speed-dependent and larger at slower gait speeds. The responses to prolonged perturbation are interpreted as step-to-step adaptations that may inform future interventions and studies on gait speed selection.