Triceps brachii muscle

Origin and insertion

The triceps brachii muscle has three heads, each of which have their own origin: 

• The long head arises from the infraglenoid tubercle of scapula, which is a rough area at the inferior margin of the glenoid fossa. Its attachment extends slightly above to the adjacent glenoid labrum and blends with the glenohumeral capsule of the shoulder joint, contributing to its stability.
• The lateral head originates from a narrow, linear ridge on the posterior surface of the humerus, just superior to the radial groove. A portion of the muscle fibers also arise from the lateral intermuscular septum. Its attachment ascends obliquely beginning from the lateral border of the humerus behind the deltoid tuberosity. From here, it extends to the surgical neck of humerus, medial to the insertion of teres minor and above the attachment for the medial head of triceps.
• The medial head is overlapped by the long and lateral head of the triceps. It has a broad origin along the entire posterior surface of the humerus inferior to the radial groove. More specifically, its attachment extends over an elongated triangular area on the humerus. The apex of the triangle is located on the medial border of the humerus above the insertion of teres major, while the base is the line that connects the medial and lateral epicondyles of the humerus. The medial head has an additional attachment to the posterior aspect of the medial and lateral intermuscular septum.

All three muscle bellies converge onto a common tendon, which inserts to the posterior aspect of the proximal surface of the olecranon of the ulna. Laterally, a band of fibres continues inferiorly over the anconeus muscle to blend with antebrachial fascia of the forearm. 

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Relations

Triceps brachii is the only muscle in the posterior compartment of the arm. The superior portion of the long head of triceps is covered by the posterior border of the deltoid muscle. The long head of triceps can be palpated as an elevation parallel and medial to the posterior border of the deltoid muscle when the elbow is extended.

On its course, the long head of the triceps descends through the wedge-shaped interval between the teres major and minor and the humerus, thereby creating three distinct anatomical spaces.

• The upper triangular space is bounded by teres minor and subscapularis superiorly, teres major inferiorly and the long head of triceps laterally. This space allows the passage of the circumflex scapular artery and vein from the axillary region to the scapular region.
• The lower triangular space, also called the triangular interval, is bounded by teres major superiorly, the long head of triceps medially and the humerus laterally. This space is a conduit between the posterior and anterior compartments of the arm, which allows passage of the radial nerve and the deep brachial vessels.
• The quadrangular space is a horizontal cleft or tunnel bordered by subscapularis and teres minor superiorly, teres major inferiorly, long head of triceps medially, and the surgical neck of humerus laterally. This space allows the axillary nerve and the posterior circumflex humeral artery and vein to pass from the axillary region to the posterior region of the shoulder.

Inferior to the origin of the lateral head, and superior to the origin of the medial head, the radial groove runs from the upper medial border of humerus downwards and laterally towards the lateral border; it contains the radial nerve and the deep brachial artery and vein.

On the medial side of the arm, the medial head of the triceps courses adjacent to the biceps brachii muscle anteriorly. The space between the medial head of the triceps and the biceps brachii forms the medial bicipital groove, which provides a passageway for the brachial artery and median and ulnar nerves. 

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Innervation

The triceps brachii is innervated by the radial nerve, which gives off a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, root value C7 innervates the long head, and root value C8 supplies the medial head.

Blood supply

The triceps brachii muscle is mainly supplied by the deep brachial artery and the superior ulnar collateral artery, which arises from the brachial artery. Additional supply to the lateral head of the triceps brachii is provided by the posterior circumflex humeral artery.

Function

The triceps and biceps brachii are the main muscles controlling the movements of the elbow. The main function of triceps brachii is extension of the forearm at the elbow joint. 

More specifically, triceps takes part in active extension, which occurs both as a result of the contraction of the triceps brachii muscle and the relaxation of biceps brachii. Thereby, triceps brachii is most active in pushing or thrusting movements, as well as supporting body weight on the hands with the elbows semi-flexed (e.g. when using arms to lift oneself up from a chair).

However, during passive extension of the forearm (e.g. when lowering a heavy object), the extensor action of the triceps is replaced by gravity, and the movement is controlled by active lengthening or eccentric contraction of the biceps brachii and other flexors of the forearm.

The medial head of the triceps is active in all forms of forearm extension, while the long and lateral head are only significantly active during extension at the elbow that occurs against resistance.

The long head of the triceps has several additional actions reflected upon its attachment points: 

• Due to its attachment on the scapula, the long head can also act on the shoulder joint, producing an extension of the arm. 
• As the attachment of the long head also blends with the glenohumeral capsule, it contributes to the shoulder joint stability. More specifically, it helps to hold the head of the humerus in the glenoid cavity and prevents its inferior displacement. 
• When the arm is extended, the long head can act on the glenohumeral capsule to pull superiorly on the humerus and produce the adduction of the arm.