Posterior Compartment of Leg
On this page
Direct answer
Calf power, venous pumping and push-off all live in the posterior compartment of the leg, arranged in two layers. Superficially, gastrocnemius (two heads from the femoral condyles), soleus (from the soleal line and posterior fibula, below a tendinous arch over the popliteal vessels) and the slender plantaris unite as the triceps surae in the tendo calcaneus — the thickest tendon in the body — gripping the middle of the posterior calcaneal surface. Deep lie popliteus, flexor hallucis longus, flexor digitorum longus and tibialis posterior, all supplied, like the superficial group, by the tibial nerve (S1–S2 for the triceps surae). The posterior tibial artery and the tibial nerve run between the layers, pass behind the medial malleolus, and give the peroneal artery, the compartment's large, fibula-hugging branch.
What you must remember
- Muscle map: superficial — gastrocnemius, soleus, plantaris (absent in about one in ten, its long tendon medial to the Achilles); deep — popliteus, flexor digitorum longus, tibialis posterior, flexor hallucis longus (crossing the other two tendons behind the ankle).
- Actions: plantarflexion and knee flexion (gastrocnemius); postural plantarflexion and the venous pump ("the peripheral heart"); popliteus unlocks the fully extended knee by laterally rotating the femur on the tibia; the deep trio plantarflexes the toes and, through tibialis posterior, supports the arch and inverts the foot.
- Neurovascular bundle: posterior tibial artery (from the popliteal, under the tendinous arch of soleus) and tibial nerve, running on tibialis posterior between superficial and deep layers, ending behind the medial malleolus where the artery's pulse is felt midway to the tendo calcaneus.
- Peroneal artery: the largest branch of the posterior tibial, descending along the medial crest of the fibula — the surgeon's landmark and the flap surgeon's pedicle.
- Tendo calcaneus: the strongest and thickest tendon; avascular at its critical zone 2–6 centimetres above the insertion, which is where ruptures occur; a retrocalcaneal bursa and the Achilles bursa cushion it against the calcaneus and skin.
- Rupture signs: sudden "kick in the calf" during badminton or cricket, a palpable gap, no plantarflexion on calf squeeze (Simmonds/Thompson test), and weak plantarflexion preserved through plantaris and the long flexors — the classic false reassurance.
- Deep vein territory: soleal venous sinuses and the calf pump make this compartment the commonest origin of leg deep vein thrombosis — pain, swelling and tenderness after immobilisation.
Two injuries, one compartment
A 35-year-old badminton player feels a shot to the back of the ankle mid-lunge — no one is behind him. He can hobble and even push the foot down feebly, so casualty sends him home with a "sprain". The missed structure is the Achilles tendon, ruptured at its avascular critical zone; the giveaway is the calf-squeeze test — squeezing the relaxed calf should plantarflex the foot; if it stays still, the tendon is gone. Plantaris, intact in most, permits that deceptive flicker of movement. The middle-aged "tennis leg" is a tear at the musculotendinous junction of the medial head of gastrocnemius, its haematoma mimicking deep vein thrombosis — ultrasound settles both. Genuine deep vein thrombosis originates in the soleal sinuses after surgery or long journeys: calf tenderness and swelling, investigated urgently because clot propagates toward the lungs.
High-yield viva angles
"Why is soleus called the peripheral heart?" — its rich venous plexus and relentless postural contractions pump blood against gravity from the legs to the heart; failure of this pump (immobilisation, varicose incompetence) is the engine of venous stasis and calf thrombosis. "What exactly does popliteus do?" — it unlocks the screw-homed extended knee by laterally rotating the femur, the first event of every flexion cycle; its tendon forms the floor of the lower popliteal fossa. And the posterior tibial pulse lies behind and below the medial malleolus — examined in every diabetic foot and vascular assessment.
Frequently asked questions
Which muscles make up the triceps surae and what is their common insertion?
Gastrocnemius (two heads) and soleus, joined by plantaris, inserting through the tendo calcaneus into the middle of the posterior surface of the calcaneus.
What is the Thompson (Simmonds) test and what does a positive result mean?
With the patient prone, squeezing the calf muscles should plantarflex the foot; failure means a ruptured Achilles tendon, even if weak plantarflexion persists via plantaris and the deep flexors.
What is the role of popliteus at the knee?
It unlocks the fully extended knee by laterally rotating the femur on the fixed tibia (medially rotating the tibia when free), initiating flexion from the screw-home position.
Which artery and nerve run between the superficial and deep posterior compartments?
The posterior tibial artery with the tibial nerve, entering beneath the tendinous arch of soleus and passing behind the medial malleolus into the tarsal tunnel.
Why is the soleus called the peripheral heart?
Its contractions pump blood from the soleal venous plexus upward against gravity, making it the main calf venous pump — the reason immobilisation of this muscle predisposes to deep vein thrombosis.