Design and evaluation of the SMApp: A stiffness measurement apparatus for ankle–foot orthoses

Abstract Ankle–foot orthoses (AFOs) are braces worn by individuals with gait impairments to support ankle motion. AFO rotational stiffness is a key mechanical property that affects gait biomechanics. However, it is unclear how design choices affect AFO stiffness and what specific stiffness ranges impact gait. This stems largely from the absence of commercial devices or established guidelines for measuring AFO stiffness. This paper details the design of the AFO Stiffness Measurement Apparatus (SMApp). The SMApp is a non-destructive, automated system that measures torque and angle as an AFO is moved through a range of motion at a set speed. Using the derived torque–angle curve, we can calculate rotational stiffness, neutral angle and hysteresis of any AFO for speeds up to 37 deg/s. The repeatability of the SMApp was evaluated between and within two operators and three testing sessions using three AFOs cycled 10 times through flexion and extension. We also measured the stiffness of three different AFOs with the SMApp and another published system in a comparative case study. The SMApp had excellent reliability with high intraclass correlation coefficients (ICC ≥ 0 . 97 ) for all measured AFO properties. The standard error of measurement (SEM) and minimum detectable difference (MDD) were comparable to published parameters from other devices for stiffness (SEM i 0.32 N m/°, MDD i 0.88 N m/°) and neutral angle (SEM i 0.30°, MDD i 0.84°). In comparison to a previously described, reliable manual stiffness measurement device, the SMApp measurements had similar stiffness, but different neutral angle values for one of three AFOs. The SMApp can be utilized in future studies for AFO material fatigue testing, to validate finite element models, and to evaluate speed effects on AFO properties.