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AN19.1-7 | Back of Leg & Sole — SDL Guide
Learning Objectives
- After completing this module, you will be able to identify and describe the major muscles of the back of the leg — their attachments, nerve supply, and actions (AN19.1)
- Trace the course, relations, and branches of the important nerves and vessels of the posterior leg including the tibial nerve and posterior tibial artery (AN19.2)
- Explain the concept of the 'peripheral heart' and the calf muscle pump mechanism in venous return (AN19.3)
- Describe the anatomical basis of calcaneal tendon rupture and apply the Thompson test clinically (AN19.4)
- Explain the factors maintaining the arches of the foot and their functional significance (AN19.5)
- Discuss the anatomical basis of flat foot (pes planus) and club foot (talipes equinovarus) (AN19.6)
- Explain the anatomical basis of metatarsalgia and plantar fasciitis with Indian clinical contexts (AN19.7)
INSTRUCTIONS
Read through each section carefully. Bold terms are key vocabulary — understand their definitions before moving on. Attempt the Self-Check questions before revealing the answer. Spend time on the Clinical Pearls as these link anatomy to real ward presentations.
References
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
Figure: Superficial Posterior Compartment: The Calf Muscles
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
CLINICAL PEARL
The Thompson (Simmonds) test is the definitive bedside test for calcaneal tendon rupture:
Method: Patient lies prone on examination table. Examiner squeezes the calf at the widest point. Normal: foot plantarflexes (tendon intact). Rupture: no movement of foot.
Why it works: Squeezing the calf compresses gastrocnemius and soleus. The mechanical force is transmitted through an intact calcaneal tendon to plantarflex the foot. When the tendon is completely ruptured, this force transmission is broken.
Anatomy of rupture site: The avascular zone 2-6 cm above the calcaneal insertion has the poorest blood supply. Repetitive microtrauma in this zone → tendon degeneration → rupture with sudden eccentric loading (e.g., pushing off to sprint).
Management note: Ruptures in patients <40 years who are active → surgical repair. Older/sedentary patients → conservative cast immobilisation. Either way, understanding the anatomy guides the choice.
SELF-CHECK — 1
A patient presents with inability to stand on his toes. Which muscle is most likely affected?
A. Tibialis anterior
B. Gastrocnemius
C. Peroneus longus
D. Extensor digitorum longus
Reveal Answer
Answer: B. Gastrocnemius
The avascular zone of the calcaneal tendon (most common site of rupture) is located:
A. At the musculotendinous junction
B. At the calcaneal insertion
C. 2-6 cm above the calcaneal insertion
D. Behind the medial malleolus
Reveal Answer
Answer: C. 2-6 cm above the calcaneal insertion