The latissimus dorsi (also known as “the lats,” Latin: musculus latissimus dorsi) is one of the widest muscles in the human body, and it belongs to the superficial back muscles. It is a large, flat triangular muscle that runs across the trunk to reach the humerus, providing movements at the shoulder joint. Besides moving 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 participates in forming the posterior axillary fold in the armpit, where it is easily palpable.
The muscle fibers of the latissimus dorsi arise from several structures, including:
- spinous processes of seventh thoracic to fifth lumbar vertebrae (T7 - L5), and the thoracolumbar fascia;
- iliac crest of the sacrum;
- lower three or four ribs;
- inferior angle of the scapula.
Based on its origins, the latissimus dorsi may be divided into four corresponding parts: vertebral, iliac, costal, and scapular part.
All fibers of the latissimus dorsi together insert into the intertubercular sulcus of the humerus via a tendon. The muscle attaches to the humerus between the pectoralis major and teres major muscles.
By acting on the shoulder joint, the latissimus dorsi provides internal rotation, adduction, and extension of the arm.
When the humerus is fixed, the latissimus dorsi may participate in trunk movements: it elevates the trunk (for example, necessary during climbing and pull-ups) or move the trunk anteriorly (useful, for example, in cross-country skiing). It also supports other muscles of the back during lateral flexion of the lumbar part of the spine.
The latissimus dorsi is also one of the accessory respiratory muscles. Contractions of the latissimus dorsi on both sides of the body compress the rib cage, therefore, facilitating expiration.
The latissimus dorsi is innervated by the thoracodorsal nerve (C6-C8), which is a branch of the brachial plexus.