Leg
Crural fascia: deep fascia enclosing the leg, continuous with the fascia lata proximally and the extensor/flexor retinacula distally; thickened anteriorly over the tibialis anterior region.
- Intermuscular septa: anterior and posterior crural intermuscular septa (connecting crural fascia to the fibula), dividing the leg into anterior, lateral, and posterior compartments; a transverse intermuscular septum further divides the posterior compartment into superficial and deep groups.
- Interosseous membrane: connects the shafts of tibia and fibula, transmitting load and providing muscle origin, analogous to the forearm's interosseous membrane.
- Anterior compartment: dorsiflexors, deep fibular nerve.
- Lateral compartment: evertors, superficial fibular nerve.
- Posterior compartment: plantarflexors, tibial nerve.
- The leg's tight fascial boundaries and limited capacity for expansion make it the classic site for compartment syndrome, most frequently affecting the anterior compartment.
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Tibialis anterior | Lateral tibial condyle, proximal lateral shaft, interosseous membrane | Medial cuneiform, base of 1st metatarsal | Deep fibular nerve (L4, L5) | Dorsiflexion, inversion |
| Extensor hallucis longus | Middle anterior fibula, interosseous membrane | Distal phalanx of great toe | Deep fibular nerve (L5, S1) | Extends great toe; assists dorsiflexion |
| Extensor digitorum longus | Lateral tibial condyle, proximal anterior fibula, interosseous membrane | Extensor expansions of digits 2–5 | Deep fibular nerve (L5, S1) | Extends lateral 4 toes; assists dorsiflexion |
| Fibularis (peroneus) tertius | Distal anterior fibula, interosseous membrane (essentially a distal slip of EDL) | Base of 5th metatarsal | Deep fibular nerve (L5, S1) | Dorsiflexion, eversion |
Deep fibular nerve:
- Branch of the common fibular nerve, arising after it winds around the fibular neck; pierces the anterior intermuscular septum/extensor digitorum longus to enter the anterior compartment.
- Runs with the anterior tibial vessels, deep to the extensor muscles.
- Motor to all anterior compartment muscles, plus extensor digitorum brevis/hallucis brevis on the dorsum of the foot.
- Sensory territory limited to a small area of skin in the first dorsal web space between the great and second toes.
Anterior tibial artery:
- Arises from the popliteal artery bifurcation (at the lower border of popliteus), passes anteriorly through a gap in the proximal interosseous membrane to enter the anterior compartment.
- Descends with the deep fibular nerve, between tibialis anterior and extensor digitorum longus (proximally) and between tibialis anterior and extensor hallucis longus (distally).
- Becomes the dorsalis pedis artery at the ankle, anterior to the ankle joint, midway between the malleoli.
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Fibularis (peroneus) longus | Head and proximal lateral shaft of fibula | Base of 1st metatarsal, medial cuneiform (tendon crosses the sole via a groove in the cuboid) | Superficial fibular nerve (L5, S1) | Eversion; assists plantarflexion; supports the transverse arch |
| Fibularis (peroneus) brevis | Distal lateral shaft of fibula | Tuberosity of 5th metatarsal | Superficial fibular nerve (L5, S1) | Eversion; assists plantarflexion |
Superficial fibular nerve:
- Branch of the common fibular nerve, descends within the lateral compartment, supplying fibularis longus and brevis.
- Pierces the crural fascia in the distal leg to become subcutaneous, continuing onto the dorsum of the foot as sensory branches.
- Sensory territory: most of the dorsum of the foot (except the first web space, supplied by the deep fibular nerve, and the lateral foot border, supplied by the sural nerve).
Superficial Group
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Gastrocnemius | Medial head: posterior femur, above medial condyle; Lateral head, posterior femur, above lateral condyle | Calcaneus, via the Achilles (calcaneal) tendon (with soleus) | Tibial nerve (S1, S2) | Plantarflexion (powerful, especially with knee extended); assists knee flexion |
| Soleus | Soleal line and posterior proximal tibia, proximal posterior fibula | Calcaneus, via the Achilles tendon | Tibial nerve (S1, S2) | Plantarflexion (primary postural/sustained plantarflexor, unaffected by knee position) |
| Plantaris | Lateral supracondylar line of femur (just above lateral gastrocnemius head) | Calcaneus (via a long, thin tendon, usually joining the medial Achilles tendon border) | Tibial nerve (S1, S2) | Weak assistance to plantarflexion and knee flexion; largely vestigial functionally |
- Gastrocnemius and soleus together form triceps surae, converging on the Achilles tendon.
- Gastrocnemius crosses both the knee and ankle (its power is reduced with the knee flexed); soleus crosses only the ankle, making it the dominant plantarflexor during standing/walking with the knee extended or flexed.
Deep Group
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Popliteus | Lateral femoral condyle (and lateral meniscus) | Posterior proximal tibia, above the soleal line | Tibial nerve (L4, L5, S1) | Unlocks the extended knee (internally rotates femur on tibia) to initiate flexion; weak knee flexion |
| Tibialis posterior | Posterior tibia, fibula, and interosseous membrane | Tuberosity of navicular, with slips to most tarsals and metatarsal bases 2–4 | Tibial nerve (L4, L5) | Inversion; supports the medial longitudinal arch; assists plantarflexion |
| Flexor digitorum longus | Posterior tibia (medial to tibialis posterior) | Distal phalanges of digits 2–5 | Tibial nerve (S2, S3) | Flexes lateral 4 toes; assists plantarflexion |
| Flexor hallucis longus | Posterior fibula, interosseous membrane | Distal phalanx of great toe | Tibial nerve (S2, S3) | Flexes great toe (powerful, important for toe-off during gait); assists plantarflexion |
- Flexor digitorum longus and flexor hallucis longus cross posterior to the medial malleolus, then decussate in the sole (FHL tendon passes deep/posterior to FDL, crossing to lie medial in the forefoot), this crossing point is sometimes called the "knot of Henry."
- Tendon order posterior to the medial malleolus (anterior to posterior, within the tarsal tunnel): Tibialis posterior, Flexor digitorum longus, posterior tibial Artery/vein, tibial Nerve, Flexor hallucis longus (mnemonic "Tom, Dick, AN'd Harry": Tibialis posterior, Flexor Digitorum longus, Artery, Nerve, Flexor Hallucis longus).
Tibial Nerve (in the Leg)
- Continues from the popliteal fossa, passes between the two heads of gastrocnemius, descends deep to soleus with the posterior tibial vessels.
- Motor to all posterior compartment muscles (superficial and deep groups).
- Continues into the foot via the tarsal tunnel, posterior to the medial malleolus.
- Full detail covered in the Peripheral Nerves section.
Posterior Tibial Artery
- Continuation of the popliteal artery after its bifurcation, descends deep to soleus, with the tibial nerve.
- Gives off the fibular (peroneal) artery shortly after its origin (via the tibiofibular trunk in most individuals), descends within the posterior compartment, close to the fibula, supplying the deep posterior and lateral compartments, and contributing to the lateral malleolar anastomosis.
- Passes posterior to the medial malleolus (within the tarsal tunnel), dividing into medial and lateral plantar arteries.
