Large propulsion demands increase locomotor learning at the expense of step length symmetry

There is a clinical interest in increasing the extent of locomotor learning induced by split-belt treadmills that move each leg at different speeds. However, factors facilitating locomotor learning are poorly understood. We hypothesized that augmenting the braking forces, rather than propulsion forces, experienced at the feet would increase locomotor adaptation and learning. To test this, forces were modulated during split-belt walking with distinct slopes: inclined (larger propulsion than braking), declined (larger braking than propulsion), and flat (similar propulsion and braking). These groups were compared using step length asymmetry, which is a clinically relevant measure robustly adapted on split-belt treadmills. Unexpectedly, the group with larger propulsion demands (i.e., the incline group) adapted faster and more, and had larger after-effects when the split-belt perturbation was removed. We also found that subjects who propelled more during baseline and experienced larger disruptions of propulsion forces in early adaptation exhibited greater after-effects, which further highlights the catalytic role of propulsion on locomotor learning. The relevance of mechanical demands on shaping our movements was also indicated by the steady state split-belt behavior, during which each group recovered their baseline leg orientation to meet leg-specific force demands at the expense of step length symmetry. Notably, the flat group was nearly symmetric, whereas the incline and decline group overshot and undershot symmetry, respectively. Taken together, our results indicate that forces propelling the body facilitate gait adaptation during split-belt walking. Therefore, interventions that increase propulsion demands may be a viable strategy for augmenting locomotor learning in individuals with hemiparesis.Key Points SummarySplit-belt walking (i.e., legs moving at different speeds) can induce locomotor learning and even improve hemiparetic gait, but little is known about how to facilitate this process.We investigated the effect of braking and propulsion forces on locomotor learning by testing young unimpaired subjects on the split-belt condition at different slopes (i.e., flat, decline, and incline), which distinctively modified these forces.Propulsion forces facilitated locomotor learning indicated by 1) greater adaptation and after-effects following split-belt walking of the inclined group, which experienced larger propulsion demands and 2) a positive correlation between individual after-effects and subject-specific propulsion during regular walking and initial steps in the split condition.Interestingly, incline and decline groups self-selected asymmetric step lengths at steady state in the split condition, challenging the prominent view that step length asymmetry is a biomarker for inefficient gait.Our results suggest that interventions augmenting propulsion demands could correct hemiparetic gait more effectively.