Ankle Foot Orthosis (AFO)
Stiffness Optimization
The method to improve treatment outcomes with orthoses in Neuromuscular diseases
Determining the Optimal AFO Stiffness: Experimental Approach
The optimal AFO stiffness can be identified using a structured experimental protocol that combines metabolic and biomechanical gait measurements. By using a modular AFO that allows the stiffness to be adjusted by simply exchanging its dorsal leaf spring, different stiffness levels can be tested within the same device. This modularity is essential for systematically identifying which stiffness provides the greatest improvement in walking performance for each individual. In the lab a variety of footplate with different sizes is available. The calf casing is made of flexible carbon to ensure its fit on different calf sizes.
The modular AFO can be seen on the picture.
Measurments required to optimize AFO stiffness
Each participant receives the stiffness-adjustable AFO. After initial fitting, all five stiffness levels ranging from very flexible (K1) to very stiff (K5) are tested. For every stiffness configuration, the following outcomes are collected:
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Walking Energy Cost (EC), measured during a 6-minute walk test (6MWT), using breath-by-breath gas analysis to calculate walking energy cost (J/kg/m).
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Gait biomechanics, including the lower limb joint angles, moments and powers recorded during a 3D gait analysis.
How to select the optimal AFO stiffness?
Reducing walking energy cost is an objective measure for improving walking efficiency and is associated with reducing fatigue and improving physical functioning. Therefore, this is the primary outcome measure in the selection algorithm. In addition, walking speed and the gait biomechanics are used. The selection procedure consists of two steps:
Step 1: Pre-selection based on walking energy cost
This step narrows the selection to only those stiffness options that provide meaningful benefit regarding walking energy cost.
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All stiffness configurations are sorted by EC from lowest (best) to highest.
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Any configuration with an EC ≥5% higher than the lowest EC is excluded.
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An exception is made when walking speed is at least 5% higher than in the lowest-EC condition, because faster walking can be clinically relevant even at slightly higher energy cost.
Step 2: Biomechanical Evaluation
The gait patterns of the remaining configurations after step 1 are assessed. The assessor should focus on relevant gait features. For people with plantarflexor weakness due to a neuromuscular disease these are the following:
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ankle angle
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ankle moment/power
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knee angle
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knee moment
The configuration judged to provide the most physiologically favourable gait pattern is selected as the optimal EVO stiffness. In case differences on the biomechanical level are considered small/irrelevant the most flexible AFO stiffness is selected. This ensures optimal usability during daily life.
