Inversion turns the sole of the foot inward, toward the body's median plane, whereas eversion turns the sole outward, away from the median plane. Both movements occur primarily at the subtalar and intertarsal joints.
The Reasoning and Mechanism
The direction of each movement must be described from the anatomical position: the body is upright, the feet point forward, and the palms face forward. Inversion raises the medial border of the foot, while eversion raises the lateral border.
Both are movements in the frontal plane around an anteroposterior axis. They are associated with the foot rather than flexion or extension at the knee or hip.
| Feature | Inversion | Eversion |
|---|---|---|
| Direction of sole | Turns medially, toward the median plane | Turns laterally, away from the median plane |
| Border of foot raised | Medial border | Lateral border |
| Plane of movement | Frontal plane | Frontal plane |
| Axis of rotation | Anteroposterior axis | Anteroposterior axis |
| Main muscles | Tibialis anterior and tibialis posterior | Fibularis longus, brevis and tertius |
| Sporting example | Sole turns inward when landing unevenly | Sole turns outward while adjusting the foot to the ground |
A common misconception is that inversion and eversion mean turning the entire leg inward or outward. Rotation of the whole leg is internal rotation or external rotation; inversion and eversion specifically describe movement of the foot.
Excessive inversion can stretch the ligaments on the lateral side of the ankle, producing the common lateral ankle sprain. Eversion injuries are generally less common because eversion is restricted by the strong medial deltoid ligament and the position of the lateral malleolus.
IB Exam Technique
For subtopic B.1.1, an examiner may ask you to identify a movement and state its plane and axis. Use precise wording: “The sole turns medially during inversion; this occurs in the frontal plane around the anteroposterior axis.” Do not confuse the direction of the sole with the direction in which the toes point.