Pronation and supination are terms often incorrectly applied to eversion and inversion. Pronation refers to the movement of the foot complex as a whole, and consists of a combination of eversion through the subtalar joint, internal rotation of the tibia, and other movements occurring across the transtarsal joints and through the forefoot. This term is most commonly to the motion of the foot in gait, and occurs through the first half of the stance phase, as the centre of pressure moves through the lateral aspect of the heel, to the medial side of the forefoot and the first metatarsal head.
– G. Robert Colborne, Ph.D
Eversion and inversion refer to rotation of the foot about its long axis. When the sole of the foot is rotated towards the transverse centre line of the body the foot is said to be inverted. When the sole of the foot is rotated away from the transverse centre line of the body the foot is said to be everted. What most people refer to as ankle flexion is rotation of the foot about it’s transverse axis as dorsiflexion and plantarflexion. Dorsiflexion is flexion of the ankle that moves the toes of the foot closer to the shin. Plantarflexion is flexion of the ankle that moves the toes of the of the foot farther away from the shin.
The rotation of the foot about its long axis as inversion and eversion is coupled through a joint of the ankle called the subtalar joint to axial vertical rotation of the tibia. Inversion of the foot is accompanied by external rotation (towards the outside of the body) of the tibia. Eversion of the foot is accompanied by internal rotation (towards the transverse centre of the body) of the tibia.
The action of the ankle joint complex is triplanar in nature in that inversion/eversion, medial/lateral vertical axial rotation of the tibia and dorsiflexion/plantarflexion of the ankle occur concurrently and are not mutually exclusive of each other. The subtalar joint of the ankle complex acts as a 1:1 torque converter in translating rotation of the foot about its long axis (inversion/eversion) into vertical axial rotation of the tibia (medial/lateral rotation).
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