Leg

The leg is the region between the knee and the ankle joints, consisting of the tibia and fibula bones. Proximally, the tibial condyles articulate with the femur to contribute to the knee joint, while the fibula does not. The tibia and fibula connect at various points, including the proximal tibiofibular joint (a plane synovial joint), distal tibiofibular joint (a fibrous joint), and via the interosseus membrane. Distally, they form the ankle joint with the talus bone (a tarsal bone).

The leg is covered by skin, superficial, and deep fascia. The deep fascia is thickened around the ankle joint to form retinacula. These retinacula prevent bowstringing of leg muscles during ankle movements and include the extensor retinacula (for anterior compartment), peroneal retinacula (for lateral compartment) and flexor retinaculum (for the posterior compartment).

Muscles of the leg are grouped into three compartments: anterior (extensor), posterior (flexor), and lateral (peroneal):

• The anterior compartment contains extensor muscles like tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis (peroneus) tertius, responsible for dorsiflexion at the ankle joint.  Additionally, the tibialis anterior produces inversion of the foot, while the peroneus tertius produces eversion, both at the intertarsal joints. Note that dorsiflexion lifts only the foot, while plantar flexion involves lifting the whole body. This explains why the muscles for plantar flexion (posterior compartment of the leg) are significantly larger than those for dorsiflexion (anterior compartment of the leg).

• The lateral compartment of leg contains the peroneus longus and peroneus brevis muscles (a.k.a. fibularis longus and fibularis brevis). These muscles originate from the lateral surface of the fibula, pass behind the lateral malleolus of the ankle, and insert into the foot. These function as evertors of the foot. The peroneus longus, in particular, swings medially around the cuboid bone and runs across the sole of the foot all the way to its medial side, contributing to the maintenance of the transverse arch of the foot.

• Muscles of the posterior compartment of the leg are divided into two groups: superficial and deep muscles. (a) The superficial group consists of the gastrocnemius, soleus, and plantaris muscles. They insert via a common Achilles tendon into the calcaneum or heel bone, causing flexion at the knee and ankle joints. (b) The deep group comprises the tibialis posterior, flexor hallucis longus, and flexor digitorum longus muscles. They pass through the tarsal tunnel underneath the flexor retinaculum and insert into the sole of the foot, enabling plantar flexion at the ankle joint. The tibial nerve and posterior tibial vessels also pass through the same tarsal tunnel to enter the sole of foot as the medial and lateral plantar nerves and vessels.

  The popliteus muscle is located deep within the leg. It originates from the lateral femoral condyle and inserts into the upper posterior aspect of the tibia, just above the soleal line. When activated, it pulls the lateral condyle of the femur and causes the femur to externally rotate on a fixed tibia, unlocking the knee joint at the beginning of flexion.

Innervation:

The sciatic nerve typically bifurcates, usually in the upper region of the popliteal fossa, into the tibial and common fibular (common peroneal) branches. The common fibular nerve (common peroneal nerve) continues its course around the fibular neck and enters the lateral compartment of the leg, where it further divides into the deep and superficial fibular (peroneal) nerves.

• The deep fibular (deep peroneal) nerve provides innervation to (a) all muscles of the anterior compartment of the leg, (b) extensor digitorum brevis (located on the dorsum of the foot), and (c) the skin between the great and second toes, where the nerve terminates.

• The superficial fibular (superficial peroneal) nerve innervates the peroneus longus and brevis muscles. It pierces the deep fascia and continues onto the dorsum of the foot to supply the skin of the anterior leg and dorsum of the foot.

• The tibial nerve innervates all muscles in the flexor compartment, including both superficial and deep groups.

Vasculature:

Every compartment of the leg has its distinct nerve and blood supply:

• The anterior compartment houses the deep peroneal nerve, along with the anterior tibial artery and vein.

• The lateral compartment is served by the superficial peroneal nerve and peroneal vessels (branches of the posterior tibial artery and vein)

• The posterior compartment is supplied by the tibial nerve and posterior tibial vessels.