Effects of stiffness-optimized ankle–foot orthoses on gait stability in people with calf muscle weakness – An exploratory study

Despite frequently using ankle–foot orthoses (AFOs), stability problems remain aproblem among people with calf muscle weakness. Optimizing AFO stiffness to minimize walking energy cost improves walking-related outcomes compared to usual care AFOs. While walking energy cost and gait stability are related, whether stiffness-optimized AFOs improve gait stability has not been investigated. We conducted a post hoc analysis involving ten adults diagnosed with a neuromuscular disease and bilateral calf muscle weakness. 3D gait biomechanics were assessed for: 1) walking with shoes only, 2) walking with usual care AFOs, and 3) walking with stiffness-optimized AFOs. Gait stability was quantified using step length and step width variability, and the mediolateral and anteroposterior margins of stability. Differences between the three conditions were analyzed using generalized estimating equations, with adjustments made for walking speed. Stiffness-optimized AFOs did not significantly affect step length variability ( p = 0.054) or step width variability ( p = 0.35) compared to shoes only or usual care AFO. The mediolateral margin of stability was significantly larger with stiffness-optimized AFOs (0.060 ± 0.007 m) compared to shoes-only (0.050 ± 0.005, p < 0.001), but not compared to usual care AFOs (0.058 ± 0.007 m). The anteroposterior margin of stability was significantly larger with stiffness-optimized AFOs (0.27 ± 0.03 m) compared to both shoes-only (0.26 ± 0.01) and usual care AFOs (0.26 ± 0.015, p = 0.032). Our preliminary results indicate that stiffness-optimized AFOs may improve anterior-posterior margins of stability, while they do not improve other stability metrics compared to usual care AFOs in people with bilateral calf muscle weakness. Future research should explore whether optimizing AFOs specifically for stability could provide additional benefits.