Summary Triceps Brachii Origin Long head: infraglenoid tubercle of scapula; Lateral head: posterior surface of humerus, superior to radial groove; Medial head: posterior surface of humerus, inferior to radial groove Insertion Proximal end of olecranon process of ulna and fascia of forearm Action Chief extensor of forearm; long head steadies head of abducted humerus Innervation Radial nerve (C6, C7, C8) Arterial supply Branches of deep brachial artery