triceps brachii

The triceps brachii muscle (Latin for 'three-headed' muscle of the arm, it is called a three headed muscle because there are three bundles of muscle, each of different origin, joining together at the elbow) is the large muscle on the back of the human upper limb. It is the muscle principally responsible for extension of the elbow joint (i.e. straightening of the arm). Though a similarly-named muscle, the triceps surae, is found on the lower leg, the triceps brachii is commonly called simply the 'triceps'. [WP,unvetted].