Anatomical terms like inversion and eversion describe the fundamental, rotational motions of the foot. These actions are essential for allowing the foot to adapt to varied terrain and are closely associated with the ankle joint. Understanding these directional movements provides insight into how the foot functions and why certain injuries occur.
Defining Inversion and Eversion
Inversion describes the movement where the sole of the foot turns inward, rotating toward the midline of the body. This action is often a component of supination, a more complex motion that also involves pointing the toes downward.
Eversion is the opposite action, where the sole of the foot turns outward, rotating away from the body’s midline. During eversion, the outer edge of the foot is lifted upward. Both inversion and eversion are rotational movements that happen around the foot’s longitudinal axis, allowing the foot to maintain balance and absorb shock on different surfaces.
The Ankle Joint: The Primary Site of Movement
While the main ankle joint (the talocrural joint) is primarily a hinge controlling up-and-down motion, inversion and eversion occur at the joints just below it. These movements primarily happen at the subtalar joint and the transverse tarsal joint. The subtalar joint, located between the talus and calcaneus bones, is the primary site for these side-to-side rotational movements.
The foot can invert significantly more than it can evert, allowing for 25 to 30 degrees of inversion but only 5 to 10 degrees of eversion. Specific muscle groups control these actions. Inversion is caused by the tibialis anterior and tibialis posterior muscles. Eversion is achieved by the contraction of the fibularis longus and fibularis brevis muscles, which run along the outer side of the lower leg.
How These Movements Relate to Ankle Injuries
The limited range of eversion compared to inversion determines the most common ankle injury. When the foot rolls excessively, the ligaments holding the joint become overstretched or torn, resulting in a sprain. An inversion sprain, where the foot rolls inward, is the most frequent type, accounting for approximately 70–85% of all ankle injuries.
This high frequency is due to the relative weakness of the lateral ligaments, such as the anterior talofibular ligament. When the foot inverts too far, these outer ligaments are the first structures to fail. Eversion sprains are far less common because the medial ligament complex, known as the deltoid ligament, is much stronger and thicker. Injuries that force the foot into extreme eversion can be severe, potentially causing bone fractures because the strong deltoid ligament may pull a piece of the bone away before it tears.