Latissimus dorsi

The latissimus dorsi (Latin: musculus latissimus dorsi), also known as “the lats,” is a large and flat triangular-shaped muscle that runs across the lower posterior trunk from the ilium, thoracic vertebrae, ribs and scapula to reach the humerus. Mainly it provides movements at the shoulder joint. It is the widest muscle in the human body. Together with the levator scapulae, trapezius and rhomboid muscles, the latissimus dorsi belongs to the superficial back muscle group.

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

Origin

Based on the origin site, the latissimus dorsi muscle 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:

• The vertebral part arises from the spinous processes of seventh to twelfth thoracic vertebrae (Th7 - Th12) and thoracolumbar fascia.
• The iliac part originates from the posterior aspect of the iliac crest.
• The costal part usually arises from the lower four ribs (9 - 12).
• Finally, the scapular part originates from the inferior angle of the scapula.

Insertion

All fibers of the latissimus dorsi merge towards the proximal end of the humerus and via a tendon insert on the intertubercular groove of the humerus. The muscle attaches to the humerus between the pectoralis major and teres major muscles.

Action

The primary functions of the latissimus dorsi muscle include internal rotation, adduction, and extension of the arm at the shoulder joint. 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 climbing, swimming, rowing and skiing muscle as it provides movements used in the mentioned activities. Together with other muscle contractions, it can elevate the trunk (necessary during climbing) or move it forward (helpful in cross-country skiing).

Besides all the mentioned functions, the latissimus dorsi muscle is also one of the accessory respiratory muscles. Bilateral muscle contractions compress the rib cage. Therefore, it assists in forced expiration and deep inspiration.

Innervation

The latissimus dorsi is innervated by the thoracodorsal nerve (C6 - C8) - a branch of the brachial plexus.

Blood supply

The latissimus dorsi muscle mainly receives arterial blood supply from the thoracodorsal artery that is a continuation of the subscapular artery. Additionally, it is supplied by the dorsal perforating branches of the ninth to eleventh posterior intercostal arteries and upper three lumbar arteries. The posterior intercostal arteries arise from the thoracic aorta, while the lumbar arteries are branches of the abdominal aorta.