Ankle-joint power and AFO deflection: implications for physiologically relevant fatigue testing conditions

Ankle-foot orthoses (AFOs) are commonly prescribed to improve gait in individuals with neuromuscular and musculoskeletal impairments; however, no standardized protocols exist to characterize their mechanical properties, including fatigue life. Although angular deflection has been identified as a key boundary condition for mechanical testing of AFOs, the biomechanical factors governing in vivo AFO deformation remain largely unknown. This study characterized AFO angular deflection and ankle-joint power during walking and activities of daily living to determine how user-device interactions influence orthotic deformation and inform physiologically relevant fatigue testing conditions. Nine healthy adults performed walking trials with and without a custom-fit, 3D-printed AFO, as well as AFO-assisted sit-to-stand, stand-to-sit, and stair-ascent tasks. Kinematic and kinetic data were collected to compute ankle-joint power and AFO angular deflection. Discrete power and deflection events were extracted to assess power-deflection relationships. Across activities, walking elicited the greatest plantarflexion deformation (p < 0.001) and the highest average dorsiflexion deformation, although dorsiflexion did not differ significantly across activities. During AFO-assisted walking, maximum plantarflexion was observed to occur during loading response rather than at toe-off (p < 0.001). No significant linear relationship was observed between maximum plantarflexion during loading response and peak power absorption during this phase (r = 0.17, p = 0.665), whereas greater peak power generation during push-off was associated with greater plantarflexion deflection at toe-off (r = 0.75, p = 0.020). These findings indicate that, in a healthy population, there is an unexpected phase shift in peak plantarflexion to loading response, suggesting that peak AFO deformation may be related to early-stance impact rather than push-off power. Accordingly, across the activities examined, these findings support the use of gait-derived, subject-specific deformation profiles to improve the physiological relevance of angular-deflection boundary conditions for fatigue testing of rigid AFO designs.