Triceps brachii muscle

The triceps brachii muscle occupies the posterior compartment of the arm. As its name implies, it comprises three distinct heads that converge to form a common tendon. This robust tendon inserts onto the olecranon process of the ulna and the deep fascia of the forearm (deep antebrachial fascia), making the triceps the primary extensor of the elbow joint.

Individual heads

1. Long head

Originating from the infraglenoid tubercle of the scapula, the long head is unique as it crosses the glenohumeral (shoulder) joint. Consequently, beyond elbow extension, it also assists with extension and adduction of the arm at the shoulder. When the arm is adducted, this head helps stabilize the humeral head within the glenoid cavity, preventing displacement.

2. Lateral head

This head arises from the posterior surface of the humerus, superior to the radial groove, blending with the lateral intermuscular septum. It is generally considered the strongest of the three heads and is particularly active during forearm extension at the elbow.

3. Medial head

Originating from the posterior surface of the humerus, inferior to the radial groove, and connecting to the medial intermuscular septum, the medial head does not cross the shoulder joint and thus has no direct action there. It is consistently active during forearm extension at the elbow,

Subanconeus

Sometimes considered a deep extension of the medial head, the subanconeus is a small muscle or muscular slip found deep to the distal triceps tendon, anterior to it. It originates from the lower humerus, just proximal to the olecranon fossa, and inserts into the posterior capsule of the elbow joint. Its proposed function is to retract the posterior elbow capsule during extension, preventing impingement.

Tendon of triceps brachii

The common triceps tendon forms as the muscle fibers of all three heads coalesce in the distal arm. The medial head contributes a deeper, often more muscular, component to this convergence, while the long and lateral heads form a more superficial, conjoined tendon. Despite a potentially bipartite appearance on imaging, the tendon functionally inserts as a single, confluent unit onto the olecranon. This broad, dome-shaped insertion spans the width of the olecranon and includes a lateral expansion continuous with the anconeus fascia and the posterior antebrachial fascia.

Innervation

The Triceps brachii is innervated by the radial nerve, carrying nerve fibers from spinal segments C7 and C8.