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AN19.1-7 | Back of Leg & Sole — SDL Guide (Part 2)

The Peripheral Heart: Calf Muscle Pump

The 'peripheral heart' is the calf muscle pump mechanism — the most important venous return mechanism in the lower limb (AN19.3).

Anatomy of the mechanism:
1. Deep veins of the calf (especially in the soleal sinuses) are surrounded by calf muscles
2. When calf muscles contract (e.g., during walking), they compress the deep veins, pumping blood proximally toward the heart
3. Venous valves in these deep veins prevent backflow during muscle relaxation
4. The superficial veins (great + small saphenous) drain into deep veins via perforating veins, also with valves

Why 'peripheral heart'?: The calf muscles functionally substitute for cardiac pumping action in the lower limb. Blood pools in the lower limb (due to gravity) when standing still, but walking activates the pump.

Clinical consequences of pump failure:
- Varicose veins: Valve incompetence → superficial vein dilatation
- Deep vein thrombosis (DVT): Prolonged immobility (long flights, post-surgery, bed rest) → stasis in soleal sinuses → thrombus formation
- Post-thrombotic syndrome: Chronic venous hypertension → leg ulcers
- Thromboembolism: DVT fragments travel to pulmonary vasculature → pulmonary embolism (PE)

Indian context: DVT is commonly encountered post-caesarean section, post-orthopaedic surgery (hip and knee replacement), and after long-haul bus/train journeys. Early mobilisation and compression stockings are the primary preventive measures.

Figure: The Peripheral Heart: Calf Muscle Pump

Figure: Anatomy of the mechanism

Figure: Clinical consequences of pump failure

Arches of the Foot

The foot has three arches that distribute body weight and absorb shock:

Medial Longitudinal Arch (MLA)
- Highest and most important arch
- Bones: calcaneus → talus → navicular → 3 cuneiforms → metatarsals 1-3
- Keystone: talus (receives body weight from tibia)
- Maintained by: Spring ligament (plantar calcaneonavicular ligament) — primary static support; Tibialis posterior tendon — dynamic support; Flexor hallucis longus — dynamic support; Plantar fascia — passive (windlass mechanism)

Lateral Longitudinal Arch (LLA)
- Lower, less mobile, bears weight more directly
- Bones: calcaneus → cuboid → metatarsals 4-5
- Maintained by: Long and short plantar ligaments

Transverse Arch
- Runs across the foot at the level of the metatarsal heads
- Maintained by: Peroneus longus (passes under cuboid to base of 1st metatarsal — ties the arch together); Deep transverse metatarsal ligaments

Importance of arches:
- Distribute weight between heel and ball of foot
- Act as shock absorbers during walking and running
- Store and release elastic energy (spring-like action)
- Failure → flat foot or plantar fasciitis

Figure: Arches of the Foot

Figure: Importance of arches

Plantar Muscles & Plantar Nerves

The sole has four layers of intrinsic muscles:
- Layer 1 (superficial): Abductor hallucis, Flexor digitorum brevis, Abductor digiti minimi
- Layer 2: Quadratus plantae (+ FDL tendons), Lumbricals
- Layer 3: Flexor hallucis brevis, Adductor hallucis, Flexor digiti minimi brevis
- Layer 4 (deep): Plantar and dorsal interossei

Medial Plantar Nerve (larger branch of tibial nerve)
- Analogous to the median nerve in the hand
- Supplies: Flexor digitorum brevis, Abductor hallucis, Flexor hallucis brevis, 1st lumbrical
- Sensation: Medial 3½ toes and corresponding sole

Lateral Plantar Nerve (smaller branch of tibial nerve)
- Analogous to the ulnar nerve in the hand
- Supplies: All other intrinsic foot muscles (quadratus plantae, remaining lumbricals, interossei, adductor hallucis)
- Sensation: Lateral 1½ toes and lateral sole

Memory aid: 'LAFF' — Lumbricals 1, Abductor hallucis, Flexor digitorum brevis, Flexor hallucis brevis → Medial plantar nerve