The latissimus dorsi muscle (Latin widest of back) covers a wide area of the back. This large, fan-shaped muscle passes from the trunk to the humerus, and acts directly on the glenohumeral joint and indirectly on the pectoral girdle (scapulothoracic joint). The latissimus dorsi extends, retracts, and rotates the humerus medially (e.g., when folding your arms behind your back, or scratching the skin over the opposite scapula).

In combination with the pectoralis major, the latissimus dorsi is a powerful adductor of the humerus, and plays a major role in downward rotation of the scapula in association with this movement. It is also useful in restoring the upper limb from abduction superior to the shoulder; hence the latissimus dorsi is important in climbing. In conjunction with the pectoralis major, the latissimus dorsi raises the trunk to the arm, which occurs when performing chin-ups (hoisting oneself so the chin touches an overhead bar) or climbing a tree, for example. These movements are also used when chopping wood, paddling a canoe, and swimming (particularly during the crawl stroke).

To test the latissimus dorsi (or the function of the thoracodorsal nerve that supplies it), the arm is abducted 90° and then adducted against resistance provided by the examiner. If the muscle is normal, the anterior border of the muscle can be seen and easily palpated in the posterior axillary fold.