Stiffness prediction | AFOstiffness

Prediction of the optimal AFO stiffness

The optimal stiffness of an ankle-foot orthosis (AFO) can be predicted by measuring the maximal ankle moment during walking without an AFO. Ankle moment, when calculated in absolute terms and not normalized to body mass, depends on the severity of weakness, body mass and walking speed. These are all factors largely affecting the optimal AFO stiffness. Hence, ankle moment can be used as a proxy measure to determine the individual optimal AFO stiffness.

Calculation of the optimal stiffness based on ankle moment

With increasing AFO stiffness, the AFO provides more moment to support the calf muscles. This AFO-moment is used to increase the ankle moment during walking. If we assume that the AFO is not taking over work from the muscles, we can calculate which AFO stiffness is required to normalize maximal ankle moment.

The required AFO stiffness is calculated using:

AFO stiffness = (healthy peak ankle moment − measured peak ankle moment) / peak ankle angle,

using a healthy reference value of peak ankle moment of1.2 Nm/kg.

Please note that this formula is only validated for people with a maximal ankle moment <0.8 Nm/kg. A higher maximal ankle moment might indicate that there is only mild calf muscle weakness or a higher passive musle force.

We demonstrated that the stiffness predicted using this formula is highly correlated with the optimal AFO stiffness. The optimal AFO stiffness was predicted with an absolute error of 1.0 Nm/degree (Waterval et al. 2025). This error is smaller compared to stiffness levels advised by AFO manufacturers (Waterval et al. 2021)