Post-stroke deficits in the step-by-step control of paretic step width

Background: Humans partially maintain gait stability by actively controlling step width based on the dynamic state of the pelvis - hereby defined as the dynamics-dependent control of step width. Following a stroke, deficits in the accurate control of paretic leg motion may prevent use of this stabilization strategy. Research Question: Do chronic stroke survivors exhibit paretic-side deficits in the dynamics-dependent control of step width? Methods: Twenty chronic stroke survivors participated in this cross-sectional study, walking on a treadmill at their self-selected (0.57 +/- 0.25 m/s; mean +/- s.d.) and fastest-comfortable (0.81 +/- 0.30 m/s) speeds. To quantify the dynamics-dependent control of step width, we calculated the proportion of the step-by-step variance in step width that could be predicted from mediolateral pelvis dynamics, and used partial correlations to differentiate the relative effects of pelvis displacement and velocity. Secondarily, we calculated the mean and standard deviation of more traditional gait metrics: step width; lateral foot placement; and mediolateral margin of stability (MoS). We used repeated measures ANOVA to test for significant effects of leg (paretic vs. non-paretic) and speed (self-selected vs. fastest-comfortable) on these measures. Results: Relative to non-paretic steps, paretic steps exhibited a weaker (p <= 0.005) link between step width and pelvis dynamics, attributable to a decreased partial correlation between step width and pelvis displacement (p <= 0.001). Paretic steps were also placed more laterally (p < 0.0001), with a larger (p < 0.0001) and more variable (p = 0.003) MoS. The only effect of faster walking speeds was a narrower step width (p < 0.0001). Significance: Pelvis displacement was less tightly linked to step width for paretic steps than for non-paretic steps, indicating a decrease in the step-by-step reactive control normally used to ensure mediolateral stability. Instead, stroke survivors placed their paretic leg farther laterally to ensure a larger MoS, behavior consistent with a greater reliance on a generalized feed-forward gait stabilization strategy.