Which muscle is deepest in the deep posterior compartment of the leg?

• A. Flexor digitorum longus
• B. Tibialis posterior
• C. Flexor hallucis longus
• D. Popliteus

Answer: (B)

In the deep posterior compartment, depth refers to how close the muscle is to the posterior surface of the leg and the tibia. This compartment contains four muscles: tibialis posterior, flexor digitorum longus, flexor hallucis longus, and popliteus. The tibialis posterior sits deepest because its muscle belly lies directly against the posterior tibia and its tendon travels closest to the bone as it runs toward the foot, lying beneath the other deep muscles as you move down the leg. The flexor digitorum longus and flexor hallucis longus tendons pass more superficially behind the medial malleolus, and the popliteus is positioned proximally near the knee, not forming the deepest layer in the distal leg. This arrangement explains why tibialis posterior is the deepest muscle in that compartment. Its action includes plantarflexion and inversion of the foot, and it helps support the medial arch.