The gastrocnemius muscle is a muscle located on the back portion of the lower leg, being one of the two major muscles that make up the calf. The other major calf muscle, the soleus muscle, is a flat muscle that lies underneath the gastrocnemius. Both the gastrocnemius and the soleus run the entire length of the lower leg, connecting behind the knee and at the heel. A third muscle, the plantaris muscle, extends two-to-four inches down from the knee and lies between the gastrocnemius and the soleus.

The gastrocnemius branches at the top behind the knee; the two branches are known as the medial and lateral heads. The flexing of this muscle during walking and bending of the knee creates traction on the femur, pulling it toward the tibia in the lower leg and causing the knee to bend. Both the gastrocnemius muscle and the soleus join onto the Achilles tendon, which is the strongest and thickest tendon in the human body. The tendon originates about six inches above the heel, running down the center of the leg to connect to the heel below the ankle.