AFOs influence joint motion and metabolic cost; how they impact muscle function, however, remains unclear. A study, published online in the September Journal of Biomechanics, investigated the impact of different AFO stiffnesses on the medial gastrocnemius muscle and Achilles tendon function using two walking speeds. Gait analysis was performed for eight unimpaired individuals. Each participant walked at slow and very slow speeds with a 3D-printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25 Nm/°, 1 Nm/°, 2 Nm/°, and 3.7 Nm/°. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify medial gastrocnemius and Achilles tendon lengths with each AFO condition. Increasing AFO stiffness increased the peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak Achilles tendon length decreased, while peak medial gastrocnemius length increased with increasing AFO stiffness. Peak medial gastrocnemius muscle activity, length, and velocity significantly decreased with slower walking speed.