Gastrocnemius muscle

Origin

Gastrocnemius originates as two heads from the femur. The medial head originates from the popliteal surface of the femoral shaft, and the posterior surface of the medial condyle.

The lateral head originates from a facet on the upper posterolateral surface of the lateral condyle of the femur, where it joins the lateral supracondylar line. Both heads also take origin from the capsule of the knee joint. The tendinous attachments of the medial and lateral head expand, covering the posterior aspect of each head with an aponeurosis. The muscle fibers arise from the anterior aspect of this aponeurosis. 

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Insertion 

At the inferior margin of the popliteal fossa, the two heads come together and join to form a single, elongated, muscle belly. This forms most of the bulge of soft tissue on the posterior leg, referred to as the calf.

The fleshy part of the muscle extends to approximately the midpoint of the calf. In the lower leg, the muscle fibers of gastrocnemius gradually form a broad aponeurosis. The aponeurosis gradually narrows, and fuses with the fibers of another deeper muscle, the soleus, to form a large tendon called the calcaneal (Achilles) tendon. The calcaneal tendon attaches to the posterior surface of the calcaneus in the foot. 

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Anatomical relations

Proximally, the lateral and medial heads of gastrocnemius form the inferior boundaries of the popliteal fossa. The tendon of biceps femoris partially covers the lateral head, and semimembranosus partially covers the medial head. For the remainder of its length, the muscle is superficial, with both bellies visible beneath the skin.

The small saphenous vein and accompanying sural nerve run along the superficial surface of the muscle, separated from it by the deep fascia. The common fibular nerve crosses the lateral head of the muscle, between it and biceps femoris. Deep to gastrocnemius are the soleus, popliteus and plantaris muscles, the popliteal vessels and the tibial nerve. The deep flexor muscles, flexor digitorum longus, flexor hallucis longus and tibialis posterior, also lie deep to gastrocnemius. 

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Innervation and blood supply

The gastrocnemius is innervated by the anterior rami of S1 and S2 spinal nerves, carried by the tibial nerve into the posterior compartment of the leg. Both medial and lateral heads of gastrocnemius are supplied by the lateral and medial sural arteries, which are direct branches of the popliteal artery.

The arteries arise in the popliteal fossa, although the level at which these arteries arise is variable, with the medial sural artery usually arising more proximally and the lateral more distally. Minor accessory sural arteries may also branch off the popliteal and superior genicular arteries. Venous drainage is through corresponding medial and lateral sural veins into the popliteal vein.

Functions

The gastrocnemius is a powerful plantar flexor of the foot at the talocrural joint. It also flexes the leg at the knee. The actions of gastrocnemius are usually considered along with soleus, as the triceps surae group. They are the chief plantar flexors of the foot. The muscles are usually large and powerful. Gastrocnemius provides the force behind propulsion for walking, running and jumping. 

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