Effects of ankle-foot orthosis stiffness on lower-limb joint work during various movement tasks in people with calf muscle weakness—an explorative study

Background Spring-like ankle–foot orthoses (AFOs) are provided to people with calf muscle weakness to improve mobility by supporting ankle push-off. During walking, a higher AFO stiffness reduces ankle work. However, other movement tasks pose other demands on the AFO. In this explorative study, we aimed to determine whether AFO presence and stiffness influence lower-limb joint-work differently during gait initiation, obstacle crossing, and step-up in individuals with calf muscle weakness. Methods Seven participants with bilateral calf muscle weakness performed gait initiation, obstacle crossing, and step-up movements in a shoes-only condition and five AFO stiffness conditions (2.8–6.6 Nm/◦). Outcomes included forward and upward impulse, and (relative) positive lower-limb joint work. Trial-level linear mixed-effects models were used for analyses, with false discovery rate correction to account for multiple testing. Results Compared with shoes-only, wearing an AFO (2.8 Nm/◦) did not affect forward or upward impulse. Absolute ankle and knee work reduced, while the relative contribution of knee work reduced and hip work increased when using an AFO. No interaction between using an AFO and movement task was found. Increasing AFO stiffness reduced absolute and relative positive ankle work ( q < 0 . 001), while no effects on impulses nor on knee or hip work were found. Impulse and joint work significantly differed between movements, but no interaction stiffness-by-movement task interaction was found. Conclusion Our explorative study indicates that an AFO of 2.8 Nm/◦ and increasing AFO stiffness further reduce absolute ankle work while no effect on impulse was found. For relative joint work our results indicate that an interaction might exist, where the effect of increasing AFO stiffness is more pronounced in activities with relatively higher ankle power demands.