Posterior Compartment

Introduction

Introduction to the Posterior Compartment of the Leg

The posterior compartment of the leg is the largest of the four leg compartments and contains the muscles responsible for plantar flexion of the ankle, flexion of the toes, and inversion of the foot. It is divided by the transverse intermuscular septum into a superficial group (gastrocnemius, soleus, plantaris) and a deep group (popliteus, flexor hallucis longus, flexor digitorum longus, tibialis posterior). The compartment is innervated entirely by the tibial nerve (L4–S3). Its major blood supply is from the posterior tibial artery and the fibular (peroneal) artery. The powerful plantar flexors, particularly the triceps surae (gastrocnemius and soleus), are crucial for walking, running, and maintaining upright posture. The tendons of the deep muscles pass behind the medial malleolus through the tarsal tunnel, accompanied by the tibial nerve and posterior tibial vessels. The compartment is clinically significant for conditions such as Achilles tendon rupture, tarsal tunnel syndrome, deep vein thrombosis, and compartment syndrome.

Study

1. Boundaries and Subdivision

The posterior compartment is bounded anteriorly by the interosseous membrane, the tibia, and the fibula. It is separated from the lateral compartment by the posterior intermuscular septum. The deep (crural) fascia encloses it posteriorly. The transverse intermuscular septum, a fascial sheet extending from the fibula to the tibia, divides the compartment into superficial and deep sub-compartments. The superficial sub-compartment contains the triceps surae and plantaris; the deep sub-compartment contains the popliteus and the three deep muscles (flexor hallucis longus, flexor digitorum longus, tibialis posterior). The neurovascular bundle (tibial nerve, posterior tibial artery, fibular artery) runs between the superficial and deep layers.

2. Superficial Group of Muscles

Gastrocnemius: Two heads – medial head from the posterior aspect of the medial femoral condyle, lateral head from the lateral femoral condyle. The two heads unite to form the lower part of the popliteal fossa boundaries. The muscle inserts via the calcaneal (Achilles) tendon into the posterior surface of the calcaneus. Action: powerful plantar flexion of the ankle, assists in knee flexion. Innervation: tibial nerve (S1, S2). It is particularly active in running and jumping.

Soleus: Broad, flat muscle deep to gastrocnemius. Origin: posterior surface of the head and upper shaft of the fibula, soleal line of the tibia, and a fibrous arch (tendinous arch of soleus) between the fibula and tibia. It joins the gastrocnemius tendon to form the calcaneal tendon. Action: plantar flexion of the ankle, especially important for postural control during standing. Innervation: tibial nerve (S1, S2). Contains a high proportion of slow-twitch (type I) fibres.

Plantaris: Small muscle with a long slender tendon. Origin: lateral supracondylar line of the femur. Its tendon passes obliquely between gastrocnemius and soleus to insert into the calcaneus (or fuse with the Achilles tendon). Action: weak plantar flexor and knee flexor; considered vestigial. Innervation: tibial nerve (S1, S2). Often absent, its tendon is sometimes used for grafting.

3. Deep Group of Muscles

Popliteus: A triangular muscle forming part of the floor of the popliteal fossa. Origin: lateral condyle of the femur and arcuate popliteal ligament. Insertion: posterior surface of the tibia above the soleal line. Action: unlocks the fully extended knee by laterally rotating the femur on the tibia (or medially rotating the tibia on the unplanted foot); also assists knee flexion. Innervation: tibial nerve (L4, L5, S1) via a branch that curves around the popliteal artery.

Flexor hallucis longus (FHL): Origin: lower two-thirds of the posterior surface of the fibula and interosseous membrane. Its tendon passes posterior to the medial malleolus, under the flexor retinaculum, and through a groove on the posterior talus and calcaneus to insert into the distal phalanx of the great toe. Action: flexes the great toe, assists plantar flexion and inversion. Innervation: tibial nerve (S2, S3). It is the most lateral of the deep muscles.

Flexor digitorum longus (FDL): Origin: posterior surface of the tibia below the soleal line. Tendon passes behind the medial malleolus, crosses the FHL tendon in the sole, and divides into four tendons that insert into the distal phalanges of the lateral four toes. Action: flexes toes 2–5, assists plantar flexion and inversion. Innervation: tibial nerve (S2, S3).

Tibialis posterior: Origin: upper half of the posterior surfaces of the tibia, fibula, and interosseous membrane (lying between FHL and FDL). Its tendon passes behind the medial malleolus (most anterior of the deep tendons) and inserts primarily into the navicular tuberosity, with slips to all tarsal bones except the talus, and to the bases of the 2nd–4th metatarsals. Action: primary inverter of the foot, assists plantar flexion, supports the medial longitudinal arch. Innervation: tibial nerve (L4, L5).

4. Neurovascular Supply

The tibial nerve (L4–S3) enters the posterior compartment by passing deep to the tendinous arch of the soleus, after traversing the popliteal fossa. It descends on the surface of the tibialis posterior, between the FHL and FDL, accompanied by the posterior tibial artery. It supplies all muscles of the compartment. At the ankle, it passes behind the medial malleolus through the tarsal tunnel, where it divides into the medial and lateral plantar nerves. It also gives off the medial calcaneal branches to the heel.

The posterior tibial artery is the larger terminal branch of the popliteal artery. It arises at the lower border of popliteus, descends with the tibial nerve, and passes behind the medial malleolus to divide into the medial and lateral plantar arteries. It gives off the fibular (peroneal) artery, its largest branch, which descends along the medial border of the fibula, supplies the lateral compartment via perforating branches, and ends as the lateral calcaneal branches. The posterior tibial pulse is palpable behind the medial malleolus.

Venous drainage is via the venae comitantes of the posterior tibial and fibular arteries, which drain into the popliteal vein. The small saphenous vein ascends superficially in the posterior calf and pierces the deep fascia to drain into the popliteal vein.

5. Clinical Relevance

Achilles tendon rupture: The calcaneal tendon is the strongest tendon in the body but is commonly ruptured during forceful push-off, often in middle-aged recreational athletes. The patient feels a 'snap' in the calf and cannot stand on tiptoe. The Thompson (Simmonds) test is positive (loss of plantar flexion when squeezing the calf). Surgical repair or conservative immobilization in plantar flexion is required.

Tarsal tunnel syndrome: Compression of the tibial nerve or its branches within the tarsal tunnel behind the medial malleolus. Causes include ganglion cysts, varicosities, and tenosynovitis. It presents with burning pain, paraesthesia, and numbness in the sole and toes, often worse at night. Tinel's sign may be positive at the tunnel.

Posterior compartment syndrome: Less common than anterior compartment syndrome but can occur after fractures, crush injuries, or vascular insult. Deep posterior compartment syndrome particularly affects the FHL, FDL, and tibialis posterior. Pain is aggravated by passive toe extension. Fasciotomy must include release of the deep posterior compartment.

Calf muscle strain ('tennis leg'): Usually a tear of the medial head of gastrocnemius, common in sports with sudden acceleration. Presents with acute calf pain and tenderness. Must be differentiated from DVT.

Posterior tibial tendon dysfunction: Dysfunction or rupture of the tibialis posterior tendon leads to loss of the medial longitudinal arch support, causing acquired adult flatfoot deformity (pes planus). It is evaluated with the single-heel rise test; the patient cannot rise onto the ball of the affected foot.

Summary

Summary of the Posterior Compartment of the Leg

The posterior compartment is the largest leg compartment, containing superficial (gastrocnemius, soleus, plantaris) and deep (popliteus, FHL, FDL, tibialis posterior) muscle groups. All are innervated by the tibial nerve (L4–S3). The triceps surae provides powerful plantar flexion via the Achilles tendon. The deep muscles flex the toes and invert the foot; tibialis posterior is the primary invertor and arch supporter. The posterior tibial artery and fibular artery provide blood supply. The tendons, nerve, and vessels pass through the tarsal tunnel. Key clinical conditions include Achilles rupture, tarsal tunnel syndrome, compartment syndrome, and posterior tibial tendon dysfunction.