Which muscle acts as a prime mover for foot inversion?

The tibialis posterior is primarily responsible for foot inversion, making it the correct choice. This muscle is located deep within the calf and is integral to the stabilization of the foot during various movements. The tibialis posterior originates from the posterior surface of the tibia and fibula and runs down to the foot, attaching to several points in the tarsal bones.

When the tibialis posterior contracts, it not only performs foot inversion by pulling the foot inward but also contributes to maintaining the medial arch of the foot. This action is crucial for balance and proper gait mechanics, especially during activities that involve walking or running on uneven surfaces.

While the tibialis anterior does assist in foot inversion as well as dorsiflexion, its primary role is not as the prime mover for inversion. The peroneus longus and flexor digitorum brevis do not play significant roles in foot inversion, with the peroneus longus being mainly involved in foot eversion and the flexor digitorum brevis primarily aiding in toe flexion. Therefore, the tibialis posterior stands out as the main muscle effectively facilitating foot inversion.