The triceps brachii (also triceps brachii muscle, latin: musculus triceps brachii) is a large three-headed muscle of the posterior muscle group of the upper arm.
The triceps brachii arises from two bones: the scapula and the humerus. The origin of the triceps brachii consists of three heads: the long, lateral and medial. Fibers of the long head of the triceps brachii originate from the infraglenoid tubercle. Fibers of the lateral head originate from the humerus above the groove of the radial nerve. And fibers of the medial head of the triceps brachii originate from the humerus below the groove of the radial nerve.
Fibers of the three heads of the triceps brachii converge to a single tendon that inserts onto the olecranon process of the ulna.
The main function of the triceps brachii is extension of the forearm. The long head of the triceps brachii participates in both extension and adduction of the upper arm.