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Gastrocnemius muscle

The gastrocnemius muscle is the big muscle that makes the bulge at the back of the calf.

This is the back of the right knee. The muscle has two heads - the medial head and the lateral head - each attaching to the side of one of the rounded ends of the femur bone. The two heads join together to form the main muscle bulk and it then attaches to the back of the foot via its tendon - the Achilles tendon.