Latissimus dorsi muscle

At a glance

English

Latissimus dorsi muscle

Latin (TA2)

Musculus latissimus dorsi

System

Muscle

Region

Back

Side

Paired, left and right

About the latissimus dorsi muscle

The latissimus dorsi is a broad, thin, triangular sheet covering the lower back and the side of the chest. Its name means the widest muscle of the back. Near the midline its upper part is overlapped by the trapezius, and laterally it sweeps up behind the arm to make up most of the back wall of the armpit.

It arises over a long line: the spinous processes of the lower thoracic vertebrae, the thoracolumbar fascia that carries it to the lumbar and sacral spines, the back of the iliac crest and the lowest ribs, and often a slip from the scapula's lower angle, which it crosses. The fibres converge and twist into a flat tendon that winds round teres major and attaches deep in the intertubercular groove, on its floor. A memory aid calls it a lady between two majors, since pectoralis major fixes to the lateral lip of the groove and teres major to the medial lip.

At the shoulder it extends, adducts and medially rotates the humerus and draws the raised arm down and back, as in swimming, rowing or a pull-up. With the arms fixed overhead it lifts the trunk towards them, and through its spinal attachments it helps extend and side-bend the lumbar spine. Its costal fibres also assist in forced breathing.

Its edges help bound two useful spaces. The lumbar triangle lies between its lower border, the external oblique and the iliac crest, and is a rare site of hernia. The triangle of auscultation lies between it, the trapezius and the medial border of the scapula, where breath sounds are heard more clearly. An occasional slip, the axillary arch, crosses the armpit superficial to the axillary artery, vein and plexus and can confuse surgeons. Because it has a single dominant artery, the muscle is widely used as a flap in reconstructive surgery, and strains, though uncommon, are seen in throwing athletes.

Attachments, action and supply

Origin

Spinous processes of the lower thoracic vertebrae, thoracolumbar fascia, posterior iliac crest, the lowest ribs and often the scapula's lower angle.

Insertion

Floor of the humeral intertubercular groove, between pectoralis major and teres major.

Action

Extends, adducts and medially rotates the arm; with the arms fixed it raises the trunk, and it assists lumbar extension, side-bending and forced breathing.

Nerve supply

Thoracodorsal nerve (C6, C7, C8).

Blood supply

Mainly the thoracodorsal artery, a branch of the subscapular artery, with segmental twigs from the posterior intercostal and lumbar arteries.