The latissimus dorsi (also known as “the lats”) is one of the widest muscles in the human body. It is a large, flat triangular-shaped muscle that runs across the lower posterior trunk between the ilium, thoracic vertebrae, ribs, scapula and humerus. It mainly provides movements at the shoulder joint. 

As the most superficial muscle of the lower back, it covers almost all back muscles, except the trapezius, which is located superior to it. Together with the teres major, this muscle forms the posterior axillary fold in the armpit, and it is a site at which the latissimus dorsi is easily palpable. Besides moving the arm, the latissimus dorsi can also move the trunk, and it functions as an accessory muscle of respiration.

Based on the origin site, the latissimus dorsi can be divided into four corresponding parts - vertebral, iliac, costal and scapular. Fibers of the latissimus dorsi arise from several structures, and each muscle part has a different origin described below.

Origin: vertebral part - spinous processes of the seventh to twelfth thoracic vertebrae (T7 - T12) and thoracolumbar fascia; iliac part - posterior aspect of the iliac crest; costal part - lower four ribs (9 - 12); scapular part - inferior angle of the scapula.

Insertion: intertubercular groove of the humerus (between the pectoralis major and teres major muscles). 

Action: it acts at the shoulder joint. The latissimus dorsi provides internal rotation, adduction and extension of the arm. When the humerus is fixed against the scapula, it helps to pull the pectoral girdle backward and may also participate in trunk movements. Sometimes it is also referred to as a climbing, swimming, rowing and skiing muscle: together with other muscles, it can elevate the trunk (essential during climbing) or move the trunk anteriorly (useful in cross-country skiing). It is also one of the accessory respiratory muscles as bilateral contractions of the latissimus dorsi compress the rib cage. Therefore, it assists in forced expiration and deep inspiration.

Innervation: thoracodorsal nerve (C6 - C8), which is a branch of the brachial plexus.

Blood supply: thoracodorsal artery (a continuation of the subscapular artery), dorsal perforating branches of the 9th - 11th posterior intercostal arteries and upper three lumbar arteries.