The gastrocnemius is one of the two main calf muscles, which sits outermost and gives the calves their distinctive shape. It has two origins, one from the inside of the knee and the other from the outside. Both join the Achilles tendon about half way down the calf, which then runs to the base of your heel. This exercise emphasises a slow descent and working through the full range to help strengthen the tendon portion and the muscle through its full range.