Triceps brachii The triceps brachii is the large muscle on the posterior aspect of the upper limb which is responsible for extension of the elbow joint. There are three origins: the lateral head from the dorsal surface of the humerus, the long head from the infraglenoid tubercle of the scapula and the medial head from the groove of the radial nerve. All three heads insert into the olecranon process of the ulna.