Image by/from OpenStax College
The latissimus dorsi (/ləˈtɪsɪməs ˈdɔːrsaɪ/) is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: latissimi dorsi) comes from Latin and means “broadest [muscle] of the back”, from “latissimus” (Latin: broadest)’ and “dorsum” (Latin: back). The pair of muscles are commonly known as “lats”, especially among bodybuilders. The latissimus dorsi is the largest muscle in the upper body.
The latissimus dorsi is responsible for extension, adduction, transverse extension also known as horizontal abduction, flexion from an extended position, and (medial) internal rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine.
Due to bypassing the scapulothoracic joints and attaching directly to the spine, the actions the latissimi dorsi have on moving the arms can also influence the movement of the scapulae, such as their downward rotation during a pull up.
The number of dorsal vertebrae to which it is attached varies from four to eight; the number of costal attachments varies; muscle fibers may or may not reach the crest of the ilium.
A muscular slip, the axillary arch, varying from 7 to 10 cm in length, and from 5 to 15 mm in breadth, occasionally springs from the upper edge of the latissimus dorsi about the middle of the posterior fold of the axilla, and crosses the axilla in front of the axillary vessels and nerves, to join the under surface of the tendon of the pectoralis major, the coracobrachialis, or the fascia over the biceps brachii. This axillary arch crosses the axillary artery, just above the spot usually selected for the application of a ligature, and may mislead a surgeon. It is present in about 7% of the population and may be easily recognized by the transverse direction of its fibers. Guy et al. extensively described this muscular variant using MRI data and positively correlated its presence with symptoms of neurological impingement.
A fibrous slip usually passes from the upper border of the tendon of the Latissimus dorsi, near its insertion, to the long head of the triceps brachii. This is occasionally muscular, and is the representative of the dorsoepitrochlearis brachii of apes. This muscular form is found in ~5% of humans and is sometimes termed the latissimocondyloideus.
The latissimus dorsi crosses the inferior angle of the scapula. A study found that, of 100 cadavers dissected:
The latissimus dorsi is innervated by the sixth, seventh, and eighth cervical nerves through the thoracodorsal (long scapular) nerve. Electromyography suggests that it consists of six groups of muscle fibres that can be independently coordinated by the central nervous system.
The latissimus dorsi is responsible for extension, adduction, transverse extension also known as horizontal abduction, flexion from an extended position, and (medial) internal rotation of the shoulder joint. It also has a synergistic role in extension (posterior fibers) and lateral flexion (anterior fibers) of the lumbar spine, and assists as a muscle of both forced expiration (anterior fibers) and an accessory muscle of inspiration (posterior fibers).
Most latissimus dorsi exercises concurrently recruit the teres major, posterior fibres of the deltoid, long head of the triceps brachii, among numerous other stabilizing muscles. Compound exercises for the ‘lats’ typically involve elbow flexion and tend to recruit the biceps brachii, brachialis, and brachioradialis for this function. Depending on the line of pull, the trapezius muscles can be recruited as well; horizontal pulling motions such as rows recruit both latissimus dorsi and trapezius heavily.
The power/size/strength of this muscle can be trained with a variety of different exercises. Some of these include:
Tight latissimus dorsi has been shown to be a contributor to chronic shoulder pain and chronic back pain. Because the latissimus dorsi connects the spine to the humerus, tightness in this muscle can manifest as either sub-optimal glenohumeral joint (shoulder) function which leads to chronic pain or tendinitis in the tendinous fasciae connecting the latissimus dorsi to the thoracic and lumbar spine.
The latissimus dorsi is a potential source of muscle for breast reconstruction surgery after mastectomy (e.g. Mannu flap) or to correct pectoral hypoplastic defects such as Poland’s syndrome. An absent or hypoplastic latissimus dorsi can be one of the associated symptoms of Poland’s syndrome.
For heart patients with low cardiac output and who are not candidates for cardiac transplantation, a procedure called cardiomyoplasty may support the failing heart. This procedure involves wrapping the latissimus dorsi muscles around the heart and electrostimulating them in synchrony with ventricular systole.
Injuries to the latissimus dorsi are rare. They occur disproportionately in baseball pitchers. Diagnosis can be achieved by visualization of the muscle and movement testing. MRI of the shoulder girdle will confirm the diagnosis. Muscle belly injuries are treated with rehabilitation while tendon avulsion injuries can be treated surgically, or with rehab. Regardless of treatment, patients tend to return to play without any functional losses.
Position of the latissimus dorsi muscle (shown in red). Animation.
Animation. Rib cage is not shown, except for 10th rib, 11th rib and 12th rib.
Clearly visible latissimus dorsi muscle of an artistic gymnast on pommel horse.
Contraction of the latissimus dorsi muscle of another artistic gymnast on still rings.