Triceps Brachii

What is the Triceps Brachii

The triceps brachii, often referred to as the triceps (‘tri’ meaning ‘three’, and ‘cep’ meaning ‘head’), is a large three-headed muscle at the back of the upper arm. Its shape may resemble a horseshoe, especially from the back. The triceps is primarily responsible for extending the elbow.

Location and Anatomy

It is one of the primary extensors of the arm, located in the dorsal or posterior compartment of the arm. The triceps is a large muscle, making up for about two-thirds of the upper arm muscles.

The three heads of the triceps are referred to as the long, lateral, and medial heads based on their location and structure.

Origin	Long head: The infraglenoid tubercle of the scapula Lateral head: The upper back of the humerus, above the radial groove Medial head: The lower 2/3rd at the back of the humerus, below the radial groove
Insertion	All three heads insert into the upper back of the olecranon of the ulna

The upper part of the triceps’ long head lies beneath the posterior part of the deltoid muscle (the common shoulder muscle). The long head can be felt by touch when you extend your elbow. It is the elevated region medial and parallel to the deltoid in the back of the arm.

Having its origin at the scapula, it runs the length of the upper arm, all the way down to the elbow. It forms 3 anatomically significant spaces, along with three other muscles — the teres major, teres minor, subscapularis, and the humerus bone. These are the upper and lower triangular spaces and the quadrangular space.

The origin points of the lateral and medial heads of the triceps have the radial groove running between them. It is an important landmark on the humerus as it holds the deep brachial artery, deep brachial vein, and radial nerve.

Medially in the arm, the triceps’ medial head lies close to the biceps brachii muscle, and the space between these two is the medial bicipital groove. This groove allows passage to the brachial artery and ulnar and median nerves.

At its origin point, a portion of the medial head is attached to the intermuscular septum, which keeps the arm’s anterior and posterior compartments separate.

Functions

Action

Straightening the forearm; long head stabilizes the shoulder joint; medial head retracts the elbow joint capsule when the elbow is extended

The primary function of the triceps brachii is the extension of the forearm at the elbow joint. When the triceps contract, the elbow straightens, and the forearm extends. Similarly, when the triceps extend, the biceps contract to fold the arm at the elbow. This shows that the triceps and the biceps are antagonistic muscles with opposite functions.

Apart from extending the forearm, the triceps also stabilizes the elbow joint during fine movements of the arm and hand, such as writing.

The muscle’s functions are divided among its three heads, based on their origin.

Long Head

Originating from the scapula’s infraglenoid tubercle, the long head also has some functions at the shoulder joint:

• When the arm is folded at the elbow, it helps in keeping the head of the humerus in place in the glenoid cavity to keep the shoulder stable. 
• The long head is also involved in both extension and adduction of the upper arm at the shoulder — for example, the swinging arm movement when walking.

Lateral Head

Originating at the back of the humerus, the lateral head does not have any effect on the shoulder joint. However, it is considered to be the strongest of the three heads of the triceps. It is active when the elbow is extended, both when the forearm is pronated or supinated.

Medial Head

Like the lateral head, the medial head originates at the back of the humerus and is only involved in extending the forearm.

Innervation

Nerve

The radial nerve; the lateral head is innervated by root value C6; the long head by root value C7; the medial head by root value C8.

A recent study has shown that the ulnar nerve may partially innervate the triceps’ medial head. Other studies have shown that the axillary nerve can partially innervate the long head.

Blood Supply

Artery

The deep brachial artery