Triceps brachii is a large muscle on the back of the upper arm, between the shoulder and elbow. It belongs to the muscular system and mainly straightens the elbow. Its three heads join towards a shared tendon, while the long head also crosses the shoulder and helps move the arm backwards.
Where is triceps brachii located?
Triceps brachii occupies most of the posterior compartment of the upper arm: the muscle group behind the humerus, the upper-arm bone. Connective-tissue partitions separate this compartment from the anterior compartment at the front, which contains biceps brachii and brachialis. The triceps extends from the shoulder region and the humerus towards the back of the elbow.
The long head of triceps brachii descends from the scapula, or shoulder blade, along the inner part of the back of the arm. Near the shoulder, it passes between teres minor above and teres major below, two muscles connecting the scapula to the humerus. The lateral head lies towards the outer side, while much of the medial head is deeper, beneath the other heads.
The radial nerve and deep artery of the arm run along the radial groove on the back of the humerus, between the lateral and medial heads of triceps. The nerve carries signals to muscles and from sensory tissues; the artery supplies blood to the posterior arm.
What are the three heads of triceps brachii?
A muscle head is a distinct portion with its own starting attachment that contributes to a common muscle. Triceps brachii has a long head, a lateral head and a medial head. Here, lateral means towards the outer side of the limb, and medial means towards the body's midline.
The heads differ chiefly in their origins, meaning the attachments from which their muscle fibres extend. Their fibres converge towards the elbow, where their combined pull extends the forearm.
| Head | Main origin | Joint or joints crossed |
|---|---|---|
| Long head | Infraglenoid tubercle of the scapula | Shoulder and elbow |
| Lateral head | Back of the humerus above the radial groove | Elbow |
| Medial head | Back of the humerus below the radial groove | Elbow |
The medial head is not simply an inner strip beside the long head. Much of it lies deep to the other heads, although part becomes superficial near the elbow, meaning closer to the surface.
Where does triceps brachii originate?
The long head arises chiefly from the infraglenoid tubercle, a bony prominence immediately below the shoulder socket on the scapula. Its attachment also includes the adjacent glenoid labrum, the rim of fibrocartilage around that socket. This scapular origin distinguishes it from the other heads, which arise from the humerus.
The lateral head arises from the posterior surface of the humerus above the radial groove. The radial groove is the channel associated with the radial nerve and deep artery of the arm. The medial head arises from the posterior humerus below this groove.
The humeral heads also have attachments to the neighbouring intermuscular septa. These are sheets of connective tissue that help separate the muscle compartments of the arm.
Where does triceps brachii insert?
An insertion is the attachment towards which a muscle usually pulls its moving part. The triceps heads converge into a broad tendon, a strong band of connective tissue that transmits muscle force to bone. The principal insertion is on the olecranon of the ulna.
The ulna is the forearm bone on the little-finger side, and its olecranon forms the prominent bony point at the back of the elbow. Because the triceps attachment lies behind the elbow's axis of movement, its pull straightens the joint. Some fibres also blend with the connective-tissue covering of the forearm.
What does triceps brachii do?
The main action is elbow extension: increasing the angle between the upper arm and forearm, or straightening the elbow. All three heads contribute to this action. Triceps also controls elbow bending when it remains active while lengthening, such as when a load is lowered gradually.
The long head has additional actions because it crosses the shoulder joint. It contributes to shoulder extension, moving the upper arm backwards, and adduction, drawing the arm towards the trunk. The lateral and medial heads do not cross the shoulder and therefore do not produce these shoulder movements directly.
Triceps acts against the elbow flexors, the muscles that bend the elbow. Opposing muscles can also work together to steady a joint rather than produce a large movement.
What nerve supplies triceps brachii?
Innervation means a structure's nerve supply, linking it to the nervous system. Triceps brachii is supplied by the radial nerve, with branches reaching its heads in the upper arm. Signals through these branches activate the muscle fibres.
The radial nerve comes from the brachial plexus, a network of nerves supplying the upper limb. It travels into the posterior arm before continuing towards the elbow and forearm. The deep artery of the arm accompanies it for part of this course and contributes to the blood supply of triceps.
How can you find triceps brachii in the 3D model?
The atlas entries associated with this article are anchored to the long head of triceps brachii, one part of the whole muscle. The corresponding side-specific structures are the long head of left triceps brachii and the long head of right triceps brachii. They should not be confused with the complete triceps on either side.
In the 3D model next to the article, rotate towards the back of the upper arm, then select and isolate the long head. Use its position between the shoulder region and elbow to orient yourself, and zoom to examine its course. Remember that its scapular origin explains why this head can act at the shoulder as well as the elbow.