Deep posterior muscles of the leg

Popliteus muscle

The popliteus muscle is the smallest of all four muscles in the deep compartment. It is also the most superior to all others, with its origin in the lateral condyle of the femur, in addition to the posterior horn of the lateral meniscus of the knee joint. The muscle extends downwards and medially over the posterior aspect of the tibia and inserts just above the soleal line on the posterior side of the tibia.

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The muscle forms the floor of the popliteal fossa and its tendon separates the lateral meniscus from the lateral ligament of the knee joint. This is the only muscle of the deep posterior compartment that acts exclusively in the knee joint.The popliteus muscle is essential for the processes of walking, standing up, and sitting down. During the closed chain gait cycle (with the foot in contact with the ground), it unlocks the knees by laterally rotating the femur on the tibia. On the other hand, during open chain movements (where the foot is not in contact with the ground), the muscle medially rotates the tibia on the femur.

Flexor hallucis longus muscle

This muscle originates from the distal two-thirds of the posterior surface of the fibula and adjacent portion of the interosseous membrane.

The muscle fibers extend through the posterior compartment of the leg and converge to form a solid tendon that passes behind the distal end of the tibia just before the ankle. The tendon then passes through a groove on the posterior surface of the talus, continues through the groove of the inferior surface of the sustentaculum tali, to finally reach its insertion on the plantar surface of the base of the distal phalanx of the hallux. The contraction of this muscle causes plantarflexion of the foot, as well as the flexion of the hallux. It also assits in inversion of the foot.

Flexor digitorum longus muscle

The flexor digitorum muscle has its origin on the medial side of the posterior surface of the tibia, inferior to the soleal line. The muscle fibers descend through the medial portion of the posterior compartment of the leg. The tendon of the muscle arises just above the ankle and enters the plantar portion of the foot by passing through the groove on the medial malleolus of the tibia. After this, the tendon deviates laterally, reaches the medial portion of the foot, and divides into four terminal tendons which insert on the plantar surfaces of the bases of the distal phalanges of the lateral four toes. The flexor digitorum longus performs flexion of the toes II to IV.

Tibialis posterior muscle

This muscle originates from the posterior surfaces of the interosseous membrane and adjacent portions of the tibia and fibula.

The fibers of the muscle descend through the posterior compartment of the leg, mostly covered with the flexor digitorum muscle laterally and the flexor hallucis longus muscle medially. The tendon of the muscle passes through the groove on the medial malleolus under the tendon of the flexor digitorum longus muscle. Then, it extends anteriorly through the medial plantar side of the foot and finally inserts onto the plantar surfaces of the navicular and  cuneiform bones, the bases of the second through fourth metatarsal bones, and variably the cuboid bone. The tibialis posterior causes inversion and plantarflexion of the foot. It also provides support for the medial longitudinal arch of the foot.