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The aim of this study was to describe the effect of an antipronation foot orthosis on motion of the heel relative to the leg and explore the individual contributions of the ankle and subtalar joints to this effect.Five subjects were investigated using invasive intracortical pins to track the movement of the tibia, talus, and calcaneus during walking with and without a foot orthosis.The antipronation foot orthosis produced small and unsystematic reductions in eversion and abduction of the heel relative to the leg at various times during stance. Changes in calcaneus–tibia motion were comparable with those described in the literature (1°–3°). Changes at both the ankle and subtalar joints contributed to this orthotic effect. However, the nature and scale of changes were highly variable between subjects. Peak angular position, range of motion, and angular velocity in frontal and transverse planes were affected to different degrees in different subjects. In some cases, changes occurred mainly at the ankle; in other cases, changes occurred mainly at the subtalar joint.The changes in ankle and subtalar kinematics in response to the foot orthosis contradict existing orthotic paradigms that assume that changes occur only at the subtalar joint. The kinematic changes due to the orthosis are indicative of a strong interaction between the often common function of the ankle and subtalar joints.