Ankle Foot Orthosis (AFO)
Stiffness Optimization
The method to improve treatment outcomes with orthoses in Neuromuscular diseases
Prediction of the optimal AFO stiffness is possible based on ankle moment
A simple biomechanical formula can estimate the optimal stiffness of an ankle–foot orthosis (AFO) with an average error of about 1 Nm/degree (Waterval et al. 2025). This level of accuracy is more precise than the stiffness ranges typically advised by AFO manufacturers.
Calculating optimal stiffness
As AFO stiffness increases, the orthosis generates a larger moment that supports the calf muscles. If we assume that the AFO supplements rather than replaces muscle function, we can estimate the stiffness required to restore the ankle moment toward healthy levels.
The required AFO stiffness can be calculated as:
AFO stiffness = (healthy peak ankle moment − measured peak ankle moment) / peak ankle angle
For the healthy reference, a peak ankle moment of 1.2 Nm/kg is used.
Correlation with experimentally optimized AFO stiffness
A strong correlation of 0.8 between a predicted required AFO stiffness and experimentally optimized AFO stiffness has been shown. This means that if the formula predicts a higher stiffness also experimentally a higher stiffness was found optimal.
Note that the formula works only in cases with an 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.
Copied from: Waterval, N. F. J., Arch, E. S., Nollet, F., & Brehm, M. A. (2025). Prediction of the optimal ankle foot-orthosis stiffness based on the peak ankle moment during walking in neuromuscular disorders. Disability and Rehabilitation, 47(20), 5395-5401.
Key take aways on how to use it in clinical practice
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The prediction has a rather large error, but is more accurate than general manufacturer recommendations (Waterval et al. 2021).
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Use the outcome of the formula as a starting point to determine which AFO most likely results in the most benefits for the individual patient
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Note that to date this formula has only been validated in people with plantarflexor weakness due to a neuromuscular disease.
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The formula does not take activities other than walking into account. People walking on sloped terrains may benefit from less stiff AFOs for example. Take this into account when providing the AFO.