Triceps brachii muscle
The triceps brachii (triceps) is a large, three-headed muscle of the upper arm, consisting of the following:
- long head: originates at the infraglenoid tubercle of the scapula. From there it courses between the teres major and teres minor and divides the axillary space into two halves.
- medial head: has its' origin at the dorsal humerus distally from the radial sulcus. Its' fibers are connected with the medial intermuscular septum.
- lateral head: originates at the dorsal humerus proximally from the radial sulcus where it is fixed to the lateral intermuscular septum.
All three heads together form a thick tendon which inserts at the olecranon of the ulna and additionally to the capsule of the elbow joint and antebrachial fascia. Under the triceps tendon lies an important bursa preventing excessive friction of the tendon with the olecranon (subtendinous bursa of triceps brachii).
The radial nerve (C6-C8), which supplies the triceps, is in close relationship with the muscle. First it runs within the radial sulcus of the humerus where it is covered by both the medial and lateral heads of the triceps. From there it courses to the crook of the arm along the lateral bicipital groove between the triceps and biceps.
Due to its' superficial course, the triceps determines the contour of the dorsal upper arm. The proximal part of the triceps is partly covered by the deltoid. Both the lateral and long heads can be easily palpated, whereas the medial head lies more deeply, as it is overlaid by the other two.
The triceps is the most important extensor muscle of the elbow. Hereby the medial head is the most active of all three. Its' antagonists (particularly the biceps and brachialis) are physiologically more powerful than the triceps, which is why we have a slight bend at the elbow when our arms are freely hanging during rest.
Furthermore, the long triceps head also crosses the shoulder joint making it the only two-jointed triceps part. During contraction it pulls the upper arm towards the trunk (adduction) and behind (retroversion). The distal fibers of the triceps fulfill another task, namely the protection of the capsule of the elbow joint during extreme extending movements.
Due to the muscle's superficial position, the triceps can be easily damaged in accidents. The most common causes are a forced flexion against the contracted triceps as well as direct blows to the elbow during physical activity. Classic symptoms include pain, swelling and a severe loss of extending power in the elbow. An actual triceps tendon rupture, on the other hand, occurs very rarely and is usually associated with a pre-damaged tendon, e.g. due to chronic bursitis, rheumatoid arthritis or Marfan syndrome, or a complete osseous tear at the olecranon.
During physical examination, the triceps serves as a reference muscle for the nerve roots C7 and C8. Clinically these are tested through the triceps tendon reflex in which a strike to the insertion tendon with a reflex hammer activates a reflexive contraction of the muscle.