Stiffness-optimized AFOs may improve anteroposterior margins of stability compared to both shoes-only and usual care AFOs, while not significantly improving other stability metrics compared to usual care AFOs in people with bilateral calf muscle weakness.
Key Findings
Results
Stiffness-optimized AFOs produced a significantly larger anteroposterior margin of stability compared to both shoes-only and usual care AFOs.
Anteroposterior margin of stability with stiffness-optimized AFOs was 0.27 ± 0.03 m
Compared to shoes-only (0.26 ± 0.01 m) and usual care AFOs (0.26 ± 0.015 m), the difference was significant (p = 0.032)
This was the only stability metric where stiffness-optimized AFOs showed improvement over usual care AFOs
Analysis used generalized estimating equations with adjustments for walking speed
Results
The mediolateral margin of stability was significantly larger with stiffness-optimized AFOs compared to shoes-only, but not compared to usual care AFOs.
Mediolateral margin of stability with stiffness-optimized AFOs was 0.060 ± 0.007 m
Shoes-only condition yielded 0.050 ± 0.005 m (p < 0.001 vs. stiffness-optimized AFOs)
Usual care AFOs yielded 0.058 ± 0.007 m, which was not significantly different from stiffness-optimized AFOs
Both AFO conditions improved mediolateral stability over shoes-only, suggesting this benefit is not specific to stiffness optimization
Results
Stiffness-optimized AFOs did not significantly affect step length variability or step width variability compared to shoes-only or usual care AFOs.
Step length variability comparison yielded p = 0.054, falling just short of statistical significance
Step width variability comparison yielded p = 0.35, showing no significant difference
Neither variability metric distinguished stiffness-optimized AFOs from usual care AFOs
Step length and step width variability were used as indicators of gait stability alongside margins of stability
Methods
The study was a post hoc analysis involving ten adults with neuromuscular disease and bilateral calf muscle weakness assessed under three gait conditions.
Sample size was ten adults diagnosed with a neuromuscular disease and bilateral calf muscle weakness
Three conditions were assessed: shoes only, usual care AFOs, and stiffness-optimized AFOs
3D gait biomechanics were used to quantify outcomes
The study is described as exploratory, and results are characterized as 'preliminary'
The stiffness-optimized AFO approach was previously investigated for minimizing walking energy cost
Discussion
The authors conclude that future research should explore whether optimizing AFOs specifically for stability could provide additional benefits beyond what energy-cost optimization achieves.
Current stiffness optimization was designed to minimize walking energy cost, not to maximize stability
The authors suggest stability-specific optimization may yield further improvements
Stability problems remain a concern among AFO users with calf muscle weakness despite current AFO use
Walking energy cost and gait stability are noted to be related but the relationship is not direct enough to assume energy-cost-optimized AFOs fully address stability
What This Means
This research suggests that ankle-foot orthoses (AFOs) whose stiffness has been fine-tuned to reduce the energy it takes to walk may also provide some stability benefits for people with weakness in their calf muscles due to neuromuscular diseases. In a small exploratory study of ten adults, researchers compared three walking conditions — shoes alone, standard prescribed AFOs, and stiffness-optimized AFOs — and measured how stable participants' gait was using four different metrics. The stiffness-optimized braces improved one key measure of fore-aft (anteroposterior) balance stability compared to both shoes alone and standard AFOs, and improved side-to-side (mediolateral) stability compared to shoes alone, though not beyond what standard AFOs already provided.
However, stiffness-optimized AFOs did not meaningfully change the variability of step length or step width, meaning the consistency of each step was not notably better with the optimized braces than with standard ones. This suggests the stability gains from stiffness optimization are real but limited in scope. Because the study had only ten participants and was conducted as a post hoc (after-the-fact) analysis rather than a pre-planned trial, the findings should be interpreted cautiously.
This research suggests that customizing AFO stiffness to reduce walking effort may carry some additional stability advantages, particularly in the forward-backward direction. It also raises the possibility that designing AFOs with stability — rather than energy efficiency — as the primary goal might produce even greater improvements. For people with calf muscle weakness who struggle with balance while walking, these findings highlight that the way a brace is tuned can matter, and that further investigation into stability-focused brace design is warranted.
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van der Steen W, Brehm M, van Duijnhoven E, Nollet F, Waterval N. (2026). Effects of stiffness-optimized ankle-foot orthoses on gait stability in people with calf muscle weakness - An exploratory study.. Journal of biomechanics. https://doi.org/10.1016/j.jbiomech.2026.113554