Ankle Foot Orthosis (AFO)
Stiffness Optimization
The method to improve treatment outcomes with orthoses in Neuromuscular diseases
Optimizing AFO stiffness improves functioning of people with calf muscle weakness
AFO stiffness directly affects walking mechanics. By affecting walking mechanics it also influences walking efficiency (walking energy cost). If the AFO stiffness is too low, the AFO does provide enough support and energy cost is minimally reduced. If the stiffness is too high, it can overly restrict natural ankle motion causing energy cost to increase. Consequently, when walking with AFOs energy cost first decreases and later increases again, demonstrating an AFO stiffness that minimizes energy cost (see figure).
When providing the AFO that minimizes walking energy cost, it was shown that walking ability, fatigue and physical functioning improved compared to a regular orthosis (Waterval et al. 2020). This shows that optimizing towards stiffness has real world benefits. To maximize the effectiveness of AFOs, it is therefore recommended to provide AFOs with a stiffness optimized for walking energy cost.
The optimal AFO stiffness differs between individuals
Individuals vary widely in muscle strength, body mass, and activity demands (walking speeds). Consequently the demands on the AFO differ between individuals. In people with limited strength left in the calf muscles more support, e.g. a higher AFO stiffness is needed. With a higher body mass and walking speed, more strain is put on the AFO and hence a higher stiffness is needed. These factors also interact with each other.
As people differ in muscle strength, body mass and walking speed, their individual optimal stiffness differs (Waterval et al. 2023). As these factors interact, it is difficult to predict the stiffness based on experience. Hence, optimizing the stiffness using experimental protocols (link) or prediciton (link) models is necessary!