The gastrocnemius muscle, sometimes called the stomach of the leg, is the strong muscle in the back of the leg that runs from just above the knee to the heel. It is similar to the soleus muscle, but instead of assisting in walking and standing, it assists in running, jumping and other quick movements of the leg. It also is partially responsible for plantar flexing of the foot at the ankle joint and flexing of the leg at the knee.
The gastrocnemius muscle and soleus muscle form the calf of the leg. They also both join onto the tendon at the bottom of the leg called the calcaneal or Achilles tendon, which is the strongest and thickest tendon in the human body.