Which muscle primarily inverts the foot?

• A. Tibialis posterior
• B. Tibialis anterior
• C. Fibularis longus
• D. Flexor digitorum longus

Answer: (A)

Inversion of the foot means turning the sole inward toward the midline. The muscle that does this most effectively is the tibialis posterior. Its fibers pull on the navicular bone and other midfoot tarsals, rotating the foot so the sole faces medially, especially when the foot is on the ground or in plantarflexion. Because its line of pull passes behind the ankle joint, tibialis posterior provides the primary inversion moment.

Tibialis anterior also contributes to inversion, but its main job is dorsiflexion of the ankle. The fibularis (peroneus) longus everts the foot (turns the sole outward) and helps with plantarflexion, not inversion. Flexor digitorum longus can assist inversion somewhat while flexing the toes, but not to the degree of tibialis posterior. So the primary inverter is tibialis posterior.