Muscle contributions to balance control, propulsion and leg swing during healthy and post-stroke walking

Human walking requires complex muscle coordination to produce important biomechanical functions such as balance, forward propulsion and leg swing. For healthy individuals, these tasks are often accomplished effortlessly. However, for individuals post-stroke, balance, propulsion and leg swing can be compromised. Thus, the overall goal of this research was to understand how healthy individuals respond to altered balance control via mediolateral foot placement perturbations and how specific muscles contribute to propulsion and leg swing deficits in individuals post-stroke. Controlling mediolateral foot placement is critical to maintaining balance in the frontal plane, but can be difficult post-stroke. Thus, we investigated how healthy individuals maintain their balance after mediolateral foot placement perturbations to compare to individuals post-stroke. We found that participants responded to medial foot placement perturbations using lateral hip and ankle strategies and lateral foot placement perturbations using a lateral ankle strategy, but did not use a plantarflexion strategy following either perturbation. Modeling and simulation analyses further revealed changes in hip and trunk muscle contributions to foot placement, suggesting a coordinated response of the trunk and bilateral hip abductor muscles. On average, changes in muscle contributions to mediolateral ground reaction forces, torso power, and frontal plane external moments were small. These results highlight the responses of healthy individuals to altered balance control via foot placement perturbations. Individuals post-stroke often experience propulsion and knee flexion deficits, leading to slow walking speeds and stiff-knee gait. These deficits may have several underlying causes. Thus, modeling and simulation analyses of individuals post-stroke were used to identify muscle contributions to propulsion deficits, including excess braking from the vasti, plantarflexor braking, low plantarflexor output and reliance on compensatory mechanisms. Moreover, higher vasti contributions to braking in pre-swing predicted lower knee flexion. While the rectus femoris and iliopsoas did not directly contribute to lower knee flexion acceleration in pre-swing compared to contributions from the vasti, in some cases, the rectus femoris absorbed more power and the iliopsoas contributed less power to the paretic leg. These results highlight the heterogeneity of the post-stroke population and the need to identify individual causes of walking deficits to improve rehabilitation outcomes.