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AN19.1-7 | Back of Leg & Sole — Part 1

CLINICAL SCENARIO

A 38-year-old cricket fast bowler from Tamil Nadu is in the emergency department with sudden severe pain in the back of his left leg during the final over of a match. He heard a 'pop', fell to the ground, and cannot stand on his toes. On examination, there is a palpable gap 4 cm above the heel. The on-call orthopaedic registrar squeezes his calf — the foot does not move.

This is a complete rupture of the calcaneal (Achilles) tendon — one of the most important clinical presentations in lower limb anatomy. By the end of this module, you will understand exactly why the Thompson test works and how the anatomy of the posterior leg makes this injury both predictable and preventable.

WHY THIS MATTERS

The posterior compartment of the leg is one of the most clinically relevant regions in the entire musculoskeletal system:

  • Plantar fasciitis is the most common cause of heel pain in adults — seen daily in primary care, especially in policemen, farmers, teachers, and nurses who stand for prolonged periods in India
  • Calcaneal tendon ruptures are surgical emergencies common in recreational athletes aged 30-50
  • Flat foot and club foot affect millions in India — flat foot is frequently seen in army/police medical examinations as a cause of rejection
  • Calf muscle pump failure causes varicose veins and deep vein thrombosis — a major post-surgical and obstetric complication
  • Surface anatomy of the tibial nerve and posterior tibial artery are tested in every clinical examination

RECALL

Before diving in, connect this topic to what you've already studied:

  • Compartments of the leg: The leg has anterior, lateral, and posterior compartments. The posterior compartment has superficial and deep layers.
  • Popliteal fossa: The tibial nerve and popliteal artery exit the popliteal fossa to enter the posterior leg
  • Bones: Tibia, fibula, calcaneus (heel bone), talus (ankle bone) — you've seen these in the osteology sessions
  • Nerve supply pattern: Most posterior leg muscles are supplied by the tibial nerve (L4, L5, S1, S2) — the tibial nerve is the larger terminal branch of the sciatic nerve
  • Basic muscle action: Plantarflexion = pointing the foot down (like pressing the accelerator); Dorsiflexion = pulling the foot up

Superficial Posterior Compartment: The Calf Muscles

The superficial posterior compartment contains three muscles that share a common insertion via the calcaneal tendon (Achilles tendon) — the thickest and strongest tendon in the body.

Gastrocnemius
- Origin: Two heads from the medial and lateral femoral condyles (above the knee joint)
- Insertion: Via calcaneal tendon into the posterior surface of calcaneus
- Nerve supply: Tibial nerve (S1, S2)
- Action: Plantarflexion of ankle + flexion of knee. Because it crosses two joints, it is most effective when the knee is extended.

Soleus
- Origin: Soleal line on posterior tibia, upper fibula, fibrous arch between them
- Insertion: Via calcaneal tendon (deep to gastrocnemius)
- Nerve supply: Tibial nerve (S1, S2)
- Action: Plantarflexion only (single-joint muscle — does NOT flex the knee). More important for sustained posture and walking than gastrocnemius.
- Clinical note: Soleus contains large venous sinuses — a common site for deep vein thrombosis (DVT)

Plantaris
- Small, rudimentary muscle with a long thin tendon
- Origin: Lateral supracondylar line of femur
- Insertion: Medial side of calcaneal tendon or separately into calcaneus
- Action: Weak plantarflexion. Clinically: its tendon is used for tendon grafting in reconstructive surgery
- Absent in ~7% of people

The Calcaneal Tendon (Achilles Tendon)
- Formed by gastrocnemius + soleus tendons, joined by plantaris
- The avascular zone lies 2–6 cm above the calcaneal insertion — this is where most ruptures occur
- A bursa (retrocalcaneal bursa) lies between the tendon and calcaneus

Deep Posterior Compartment: The 'Tom, Dick, And Harry' Muscles

The deep posterior compartment has four muscles. Three pass posterior to the medial malleolus in a consistent order remembered as 'Tom, Dick, ANd Very Nervous Harry':

Popliteus
- Origin: Lateral surface of lateral femoral condyle
- Insertion: Posterior surface of tibia above the soleal line
- Nerve: Tibial nerve (L4, L5, S1)
- Action: 'Unlocks' the knee — internally rotates the tibia on the femur to begin knee flexion from the fully extended (locked) position. Crucial for starting to walk.

Tom = Tibialis Posterior
- Origin: Posterior surfaces of tibia and fibula, interosseous membrane
- Insertion: Tuberosity of navicular + plantar surfaces of cuneiforms and metatarsal bases 2-4
- Action: Plantarflexion + inversion (most important invertor of the foot)
- Clinical: Tibialis posterior tendon dysfunction → flat foot in adults

Dick = Flexor Digitorum Longus (FDL)
- Origin: Posterior tibia
- Insertion: Bases of distal phalanges of lateral four toes
- Action: Flexes toes + assists plantarflexion. Grips ground during toe-off phase of gait.

And = (Nothing — the 'And' is the mnemonic filler)

Very Nervous = Vessels and Nerve (tibial nerve + posterior tibial artery)
- These pass behind the medial malleolus in the tarsal tunnel

Harry = Flexor Hallucis Longus (FHL)
- Origin: Lower two-thirds of posterior fibula
- Insertion: Base of distal phalanx of great toe
- Action: Flexes great toe + assists plantarflexion. Most important for push-off in gait and dancing.
- Clinical: 'Dancer's tendinitis' — stenosing tenosynovitis of FHL in classical dancers

Tibial Nerve & Posterior Tibial Artery

Tibial Nerve (larger terminal branch of sciatic nerve, L4-S3)
- Enters the posterior leg through the popliteal fossa
- Runs between the superficial and deep compartments, deep to the soleus
- Passes posterior to the medial malleolus in the tarsal tunnel (flexor retinaculum)
- Divides into medial plantar nerve and lateral plantar nerve in the sole
- Motor: All muscles of the posterior leg + all intrinsic foot muscles (via plantar branches)
- Sensory: Posterior leg (sural nerve branch), sole of foot
- Clinical: Tibial nerve compression in the tarsal tunnel → tarsal tunnel syndrome (tingling/numbness in sole + weakness of intrinsic foot muscles)

Posterior Tibial Artery (larger terminal branch of popliteal artery)
- Accompanies the tibial nerve throughout
- Peroneal (fibular) artery is its largest branch — supplies lateral compartment and fibula
- At the tarsal tunnel: divides into medial and lateral plantar arteries
- Clinical pulse point: Palpable posterior to medial malleolus — used to assess peripheral vascular disease (PVD) and diabetes-related ischaemia
- Absent or poorly palpable in ~10% of normal individuals