triceps, any muscle having three heads, or points of attachment, but especially the triceps brachii at the back of the upper arm. One head originates on the shoulder blade and two on the upper-arm bone, or humerus. Uniting part of the way down the arm, the heads swell into the belly, or muscle proper. This tapers to a tendon that rounds the elbow and attaches to the ulna, the larger of the two forearm bones. Since contraction of the triceps straightens the arm, the muscle is called an extensor. It also helps lock the elbow when the forearm pushes forward against resistance. The triceps works in coordination with a flexor muscle, the biceps brachii of the upper arm.