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Gastrocnemius Muscle – Origin, Nerve Supply, Function

Gastrocnemius Muscle also called leg triceps, large posterior muscle of the calf of the leg. It originates at the back of the femur (thigh bone) and patella (kneecap) and, joining the soleus (another muscle of the calf), is attached to the Achilles tendon at the heel. The action of the gastrocnemius pulls the heel up and thus extends the foot downward; the muscle provides the propelling force in running and jumping.

The muscle is named via Latin, from Greek γαστήρ (gaster) ‘belly’ or ‘stomach’ and κνήμη (knḗmē) ‘leg’, meaning ‘stomach of leg’ (referring to the bulging shape of the calf).

Origin of Gastrocnemius Muscle

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 lateral 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 fibres arise from the anterior aspect of this aponeurosis.

Insertion of Gastrocnemius Muscle

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 fibres of gastrocnemius gradually form a broad aponeurosis. The aponeurosis gradually narrows and fuses with the fibres 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 is 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 the gastrocnemius.

Innervation and Blood Supply

The gastrocnemius is innervated by the ventral 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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Regional anatomy

The gastrocnemius muscle has two heads: medial and lateral. The medial head originates on the posterior surface of the femur superior to the medial condyle and posterior to the insertion of the adductor magnus muscle.
The lateral head originates in the lateral epicondyle of the femur. Both insert into a long common muscle aponeurosis which forms the Achilles tendon with the aponeurosis of the soleus.
The proximal part of the muscle forms the lower triangle of the popliteal fossa, the upper triangle being formed by the diverging hamstring muscles.
The muscle bellies extend from the popliteal fossa to the mid to distal third of the leg.
The plantaris muscle and tendon lie between the medial gastrocnemius and the soleus muscle and can be used to clearly identify the plane of separation.
The medial and lateral head run side by side and are separated by a fibrous septum.
The gastrocnemius muscles contribute to plantar flexion of the foot through their pull on the Achilles tendon. These muscles also contribute to knee flexion.

Arterial anatomy of the region

The popliteal artery is the continuation of the femoral artery. It enters the diamond-shaped popliteal fossa medially and exits it laterally. From proximal to distal, the popliteal artery gives off:

A large lateral branch that supplies the vastus lateralis, the biceps femoris, and the lateral aspect of the distal third of the thigh. It anastomoses with the fourth perforating artery and constitutes an important chain of anastomoses between profunda and popliteal arteries. The medial and lateral superior genicular arteries, which curve around both femoral condyles.
The sural arteries, one medial and one lateral, which, respectively, supply the medial and the lateral head of the gastrocnemius muscle. Both arteries arise superior to the level of the articulation of the femur with the tibia. Occasionally, the popliteal artery gives off a common sural artery that then bifurcates into a medial and lateral branch. More than one medial sural artery has been reported in a minority of cases.
The middle genicular artery, which supplies the posterior cruciate ligament.
The medial and lateral inferior genicular arteries, which arise in the apex of the lower triangle and curve around the tibial condyles.
The popliteal artery leaves the popliteal fossa by passing beneath the fibrous arch of the soleus and immediately divides into the anterior and posterior tibial arteries.

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