Triceps is the main muscle in the posterior compartment of the arm. Its name is derived from the presence of three 'heads': long, medial and lateral.
The long head arises from the infraglenoid tubercle at the superior end of the axillary border of the scapula. The lateral head originates from a linear origin superior to the radial groove and extending from the surgical neck of the humerus to the point of insertion of deltoid. The medial head has a broader origin: the whole of the posterior surface of the humerus on the inferomedial side of the radial groove and both of the intermuscular septae. The radial nerve and profunda brachii vessels lie between the lateral and medial heads.
The three heads form a flattened tendon that inserts into the posterosuperior part of the olecranon process of the ulna. Immediately proximal to this insertion, an bursa - the olecranon bursa - is interposed between triceps tendon and olecranon. Some fibres pass distally to insert into the deep fascia of the forearm. A few fibres from the medial head are inserted into the posterior part of the capsule of the elbow; this prevents the capsule from being trapped between olecranon and olecranon fossa during extension.
Triceps is innervated by the radial nerve (C6-C8). Four branches arise from the nerve very proximally: each head receives one branch apart from the medial head which receives two branches.
The vascular supply to triceps is the profunda brachii artery.
The actions of triceps are:
- main extensor of the elbow
- long head provides stability when the shoulder joint is abducted
- weak extension at shoulder