Force-field perturbations and muscle vibration strengthen stability-related foot placement responses during steady-state gait in healthy adults.

Mediolateral gait stability can be maintained by coordinating our foot placement with respect to the center-of-mass (CoM) kinematic state. Neurological impairments can reduce the degree of foot placement control. For individuals with such impairments, interventions that could improve foot placement control could thus contribute to improved gait stability. In this study we aimed to better understand two potential interventions, by investigating their effect in neurologically intact individuals. The degree of foot placement control can be quantified based on a foot placement model, in which the CoM position and velocity during swing predict subsequent foot placement. Previously, perturbing foot placement with a force-field resulted in an enhanced degree of foot placement control as an after-effect. Moreover, timed muscle vibration enhanced the degree of foot placement control whilst the vibration was applied. Here, we replicated these two findings and further investigated whether Q1) timed muscle vibration leads to an after-effect and Q2) whether combining timed muscle vibration with force-field perturbations leads to a larger after-effect, as compared to force-field perturbations only. In addition, we evaluated several potential contributors to the degree of foot placement control, by considering foot placement errors, CoM variability and the CoM position gain (β) of the foot placement model, next to the R measure as the degree of foot placement control. Timed muscle vibration led to a higher degree of foot placement control as an after-effect (Q1). However, combining timed muscle vibration and force-field perturbations did not lead to a larger after-effect, as compared to following force-field perturbations only (Q2). Furthermore, we showed that the improved degree of foot placement control following force-field perturbations and during/following muscle vibration, did not reflect diminished foot placement errors. Rather, participants demonstrated a stronger active response (higher β) as well as higher CoM variability.