The gastrocnemius, the most prominent muscle of the lower leg, gives the calf its curvy shape. On athletes, the gastrocnemius may appear as two bulbous shapes that meet in the middle of the lower leg to form an inverted "V" shape. In tandem with the hamstrings, the gastrocnemius bends the knee. It also joins with the soleus at the Achilles tendon to move the ankle.
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The gastrocnemius has two heads. From a rear view, the lateral head lies toward the outside of the leg. The medial head lies toward the inside of the leg or the midline of the body. The two heads combine to form the belly of the muscle. The muscle fibers of the gastrocnemius are mostly fast-twitch fibers. Fast-twitch fibers activate in short, explosive movements, so the gastrocnemius is most active during movements such as jumping or sprinting.
Origin and Insertion
The two heads of the gastrocnemius originate at the long, thick bone of the upper leg known as the femur. Two knobby protrusions found at the bottom of the femur are called condyles. The lateral head of the gastrocnemius originates from the inner, back surface of the lateral (outer) condyle, and the medial head originates from the inner, back surface of the medial (inner) condyle. The gastrocnemius inserts at the calcaneous bone of the heel of the foot by way of the Achilles tendon.
Muscles insert to bone through their tendons. Muscle bundles are surrounded by filmy connective tissues. These connective tissues extend from the muscle and gather to form tendons. Tendons are denser and more fibrous than muscle. The tendon of the gastrocnemius is the Achilles. It is visible at the heel of the foot. The Achilles tendon gets its name from the Greek god, Achilles, who was killed by an arrow shot through his heel, the only vulnerable part of his body.
Tip Toe in High Heels
The gastronemius passes over two joints, the knee and the ankle. It coordinates with other muscles to move each of these joints. Along with the soleus muscle, the gastrocnemius plantar flexes (straightens) the ankle in movements such as pointing the toes or rising onto "tip toe." By forcing the ankle into plantar flexion, high heel shoes contract the gastrocnemius to make it more pronounced. The gastrocnemius and the hamstrings flex (bend) the knee in activities such as running or going up stairs.
The sister muscle of the gastrocnemius is the soleus. It is more pronounced toward the bottom of the calf. The soleus and gastrocnemius work in tandem to plantar flex the ankle. The Soleus originates under the gastrocnemius at the bones of the lower leg, the tibia and fibula. It extends below the gastrocnemius until the connective tissues of the two muscles combine to form the Achilles tendon.