The gastrocnemius is a powerful muscle that is in the back part of the lower leg (the calf). It runs from its 2 heads just above the knee to the heel, and is involved in standing and walking. It forms the Achilles tendon with the soleus muscle and some anatomists consider them to be a single muscle, the triceps surae. Its name is derived from the Greek, gastroknemia, "the belly of the leg" or calf.
The gastrocnemius is located with the soleus in the superficial posterior compartment of the leg. It originates from the posterior (back) surfaces of the distal head of the femur. Its other end forms a common tendon with the soleus muscle; this tendon is known as the calcaneal tendon or Achilles tendon and inserts onto the posterior surface of the calcaneus, or heel bone.
Deep to the gastrocnemius (farther from the skin) is the soleus muscle. The plantaris muscle and a portion of its tendon run between the two muscles, whish is involved in "unlocking" the knee from the standing position. On the other side of the fascia are the tibialis posterior muscle, the flexor digitorum longus muscle, and the flexor hallucis longus muscle, along with the posterior tibial artery and posterior tibial vein and the tibial nerve. Since the anterior compartment of the leg is lateral to the tibia, the bulge of muscle medial to the tibia on the anterior side is actually the posterior compartment. The soleus is superficial midshaft of the tibia.
The action of the calf muscles, including the gastrocnemius, is to plantar flex the foot (that is, they increase the angle between the foot and the leg). It also is a flexor of the knee joint as it is attached to the back of the femur. They are powerful muscles and are vital in walking, running, and dancing. The gastrocnemius does not play as much of a part in maintaining posture as the soleus as excessive contraction could cause the knee to flex. In upright posture, it is responsible for pumping venous blood back into the heart from the periphery, and is often called the peripheral heart or the sural (tricipital) pump.
The gastrocnemius is innervated by the tibial nerve from the sciatic, specifically, nerve roots S1�S2.