A Method for Quantifying Stiffness of Ankle-Foot Orthoses Through Motion Capture and Optimization Algorithm

Adjustments of Ankle-Foot Orthosis (AFO) stiffness is commonly prescribed for people with neurologically impaired to improve walking. It is important to quantify AFO stiffness levels to provide consistent patient-specific settings. Current AFO stiffness measurement methods require bulky, complex designs, and often permanent modification of the AFO. To address these issues, we propose the Ankle Assistive Device Stiffness (AADS) test method, a simple design jig using motion capture system and musculoskeletal analysis software (OpenSim). An AFO with three different but known dorsiflexion stiffness settings was installed to verify the AADS test method. Reflective markers were attached to the AFO while it was placed on force plates and four operators dorsiflexed the AFO with each stiffness condition, five times each. The collected marker trajectory data were imported to OpenSim to calculate AFO dorsiflexion angle using inverse kinematics. Then a static optimization algorithm was used to identify external forces from operators and the AFO torque best matching the experimentally collected ground reaction force data. Estimated AFO moments were compared within the operators’ trials and with theoretically calculated AFO moments to evaluate the accuracy of AADS tests. The AADS test results were repeatable through multiple trials and across operators. In low stiffness conditions, the AADS test had greater stiffness than actual results due to the friction of the AFO joint. As the spring strength increased, the AFO stiffness measured by AADS test was lower compared with actual stiffness due to the deformation of test AFO shells. The overall percent error between the theoretical and experimental stiffness was within ±6%. Moreover, the AADS test had high precision among the different operators and trials. AADS allows anyone with access to a gait lab quick and reliable AFO stiffness quantification. This is important to clinical practices, supporting patient-specific prescription and contributing to future research studies that require AFO stiffness.