Ankle Foot Orthosis (AFO)
Stiffness Optimization
The method to improve treatment outcomes with orthoses in Neuromuscular diseases
Effects of AFO stiffness
(English subtitles available)
What is AFO stiffness?
Ankle-foot orthosis (AFO) stiffness is the resistance to ankle motion. AFO stiffness is measured in Newton-meters per degree (Nm/degree). This unit indicates how much torque (rotational force) is required to move the ankle by one degree. Higher stiffness means more resistance to movement, offering greater support but less flexibility.
How is AFO stiffness measured?
AFO stiffness can be measured by various devices. In our research the BRUCE device is used. To measure AFO stiffness a rotational force (in Nm) is applied to the AFO while the resulting angular displacement (in degrees) is recorded. The stiffness is calculated by dividing the applied torque by the angular displacement (unit Nm/degree). This assumes a linear stiffness among the range of motion.
Example of AFO stiffness measurement (copied from Bregman et al. 2012 (link)
The effects of AFO stiffness on gait biomechanics
The stiffness of an AFO determines the effect of the AFO on gait kinematics and kinetics. Results shown below are based on a study with 37 people with calf muscle weakness. The most important effects of an increase in stiffness are:
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a reduction in ankle range of motion
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an increase in maximal ankle moment
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a reduction in maximal ankle power
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a more extended knee angle in stance
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a higher external knee extension moment in stance
A graphical representation of the effects of AFO stiffness on gait. The red arrow indicates the direction of change with increasing stiffness (copied from Waterval et al. 2020 (link))