Triceps Brachii Muscle Anatomy

Location & Overview

The triceps brachii, commonly known simply as the “triceps,” is a prominent elbow extensor situated on the posterior aspect of the upper arm. It derives its name from Latin, with ‘brachii’ meaning ‘of the arm’, while ‘triceps’ indicates its three-headed structure. The triceps brachii has three heads: a long head, a lateral head, and a medial head, shown in the images below. These heads work in close proximity to each other and give the triceps brachii its distinctive horseshoe-like shape.^[1,2]

The triceps brachii is located on the posterior aspect of the humerus. It spans from the shoulder to the elbow, sharing the upper arm region with muscles such as the biceps brachii, which is anteriorly located, and the brachialis, which is deep to the biceps brachii. Proximally, it is related to the deltoid muscle, which forms the rounded contour of the shoulder.^[1]

Because of its size, this muscle occupies a substantial portion of the arm’s posterior aspect. Its lateral head contributes to the muscle’s outer rounded appearance, while the long head, more medially placed, forms part of the muscle’s overall bulk. The medial head, although less visible, still adds to the mass and strength of the triceps brachii.^[1]

Origin & Insertion

The triceps brachii’s distinctive three-headed structure gives it three origin points, each associated with a different part of the upper arm’s skeletal structure. The muscle then converges to a single common tendon that attaches to the olecranon process of the ulna.^[3,4]

The triceps’ long head originates on the infraglenoid tubercle of the scapula. This landmark is a small protrusion on the lower part of the scapula’s lateral aspect (the side of the shoulder blade facing away from the body). This is the only head of the triceps that crosses the shoulder joint, allowing it to act on the shoulder joint.^[3,4] This is further discussed in the actions section.

The lateral head originates above the radial groove. The radial groove is a shallow depression that runs obliquely down the posterior aspect of the humerus. This groove provides a pathway for the radial nerve and the deep branch of the brachial artery.^[3,4]

The medial head of the triceps originates just below the radial groove. This head is mostly concealed by the overlying lateral and long heads, but it still contributes significant power to the triceps muscle.^[3,4]

The fibers of these three triceps heads converge to form a single tendon, a tough band of connective tissue. This tendon inserts onto the olecranon process of the ulna. The olecranon process of the ulna is a large, curved bony prominence that can easily be felt on the back of the elbow. The ulna is one of the two bones of the forearm, with the olecranon process being the prominent point of the elbow that sticks out when you bend your arm.^[3,4] Research by Madsen et al. (2006) proposed that “The medial head of the triceps has a tendon that is distinct from, and deep to, the common tendon of the long and lateral heads”, suggesting that the medial head has an additional deep attachment.^[2]

The combination of these three diverse origin points and the shared insertion point on the olecranon of the ulna results in a versatile muscle capable of executing a range of arm movements at both the elbow and shoulder, which are discussed further in the next section.

Action & Function

All heads of the triceps brachii can assist in extension of the elbow. However, because the long head crosses the shoulder joint, it can also assist in adduction and extension of the arm at the shoulder joint.^[5,2]

The primary action of the triceps brachii as a whole is to extend the elbow joint. When this action occurs, it straightens the arm, counteracting the biceps brachii muscle’s action of bending the arm at the elbow.^[3,4] Research by Kholinne et al. (2018) proposed that “each head of the triceps brachii has a different pattern of force and activity during different shoulder elevations. The long head contributes to elbow extension more at shoulder elevation and the medial head takes over at 90° and above of shoulder elevation”, illustrating the complex interaction of the heads of the triceps brachii.^[5]

The three heads of the triceps do not always act in the same way. While they all contribute to elbow extension, the long head, because of its unique attachment to the scapula, has additional functions. Since it crosses the shoulder joint, it also assists in adduction and extension of the arm at the shoulder joint. The long head also plays a role in stabilizing the shoulder joint.^[3,1]

Innervation

The three heads of the triceps brachii muscle are innervated by the radial nerve, specifically the fibers from the sixth, seventh, and eighth cervical nerve roots, commonly represented as C6, C7, and C8.^[1,6]

The radial nerve is one of the five main nerves of the brachial plexus. The brachial plexus is a complex network of nerves that originate from the cervical region of the spinal cord. The C6, C7, and C8 nerve roots emerge from the spinal cord in the neck region, joining together with the first thoracic root (T1) to form the brachial plexus (along with C5). This intricate network travels down the neck, under the collarbone, and into the armpit, branching out to provide innervation to the entire upper limb.^[1,6]

The radial nerve arises from the brachial plexus, extending along the posterior aspect of the upper limb. After passing around the humerus in the ‘spiral groove’, a groove along the upper arm bone, the radial nerve reaches the posterior compartment of the arm, where it gives off branches to the triceps brachii.^[1,6]

Blood Supply

Blood is supplied to the triceps brachii muscle primarily via two arteries: the deep brachial artery (also known as the profunda brachii artery) and the superior ulnar collateral artery.^[3,7]

The deep brachial artery is the primary supplier of blood to the triceps brachii muscle. This artery originates from the brachial artery, which is a continuation of the axillary artery beyond the lower border of the teres major muscle. From its origin, the deep brachial artery courses posteriorly between the long and medial heads of the triceps brachii. It travels along the radial nerve in the radial (spiral) groove of the humerus, supplying blood to all three heads of the triceps brachii muscle.^[3,7]

The superior ulnar collateral artery, a branch of the brachial artery, also contributes to the blood supply of the triceps brachii, particularly the medial head. It travels with the ulnar nerve posterior to the medial epicondyle of the humerus, providing additional blood flow to the triceps brachii muscle.^[3,7]

In addition to these primary arteries, the long head of the triceps brachii also receives blood from the posterior circumflex humeral artery. This artery arises from the axillary artery, typically beyond the lateral border of the pectoralis minor muscle, and courses posteriorly around the surgical neck of the humerus through the quadrangular space with the axillary nerve. The posterior circumflex humeral artery provides branches that supply the shoulder joint and the surrounding muscles, including the deltoid, teres major, teres minor, and, importantly, the long head of the triceps brachii.^[8]