Muscle · Lower leg

Gastrocnemius

Musculus gastrocnemius

The muscle that gives the calf its shape — two bellies that start on the back of the thigh bone, just above the knee, and merge into the Achilles tendon at the heel. Because it crosses both the knee and the ankle it is the springy, fast part of the calf: it snaps the heel down for push-off when you walk, run or jump, and it assists knee bending. That double-joint arrangement is also why it is one of the muscles most reliably found short and tight — a stiff gastrocnemius steals ankle bend, and the foot and Achilles end up absorbing what the ankle could not. It is the classic site of a sudden 'someone kicked me in the calf' strain.