Latissimus Dorsi Muscle

The latissimus dorsi muscle (also known as "the lats" or "the lats") is the largest muscle in the human body. Except for the trapezius, it is relatively thin and covers almost all back muscles at the posterior trunk.

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The latissimus dorsi, like the levator scapulae, trapezius, and rhomboid muscles, is part of the superficial layer of the extrinsic back muscles.

The latissimus muscle is a member of the scapular motion muscle group. This muscle has the ability to pull the inferior angle of the scapula in various directions, resulting in shoulder joint movements such as internal rotation, adduction, and extension of the arm. Furthermore, it is an accessory respiratory muscle as well as one of the main spine stabilisers during its various movements.

Latissimus Dorsi Origin and Insertion

The latissimus dorsi muscle is located in the lower thoracic and lumbar regions of the back. This muscle can be divided into four parts based on its origin:

The thoracolumbar fascia and the spinous processes of the 7th to 12th thoracic vertebrae form the vertebral part. The coastal part has origins from the ninth to the twelfth ribs. Iliac section: beginning at the iliac crest. Starting from the inferior angle of the scapula, the scapular part is performed (inconstant)

The fibres all converge on the proximal humerus. Upper vertebral and scapular fibres follow a nearly horizontal path, lower vertebral and iliac fibres follow an oblique path, and costal fibres follow a nearly vertical path. The fibres turn spirally around the teres major muscle at this level, with the lower part of the latissimus dorsi inserting proximally at the humerus and the upper part more distally. All fibres attach to the floor of the humeral intertubercular sulcus between the pectoralis major and teres major.

The following mnemonic can help you remember the relationship of the latissimus dorsi, pectoralis major, and teres major muscles as they insert in the intertubercular sulcus: 'Lady in the middle of two majors':Latissimus dorsi (Lady), Teres major and pectoralis major are two majors.

Relations

The latissimus dorsi muscle covers the serratus posterior muscles and is found superficially in the lower two-thirds of the trunk. The teres major muscle is located above the latissimus dorsi. These two muscles' adjoining fibres are joined but separated by a bursa towards their humeral attachments. The latissimus dorsi and teres major form the posterior axillary fold by spanning the space between the scapula and the proximal humerus. When the arm is adducted against resistance, the fold is accentuated. During this movement, the entire inferolateral border of the latissimus dorsi can be traced to its attachment to the iliac crest.

The medial margin of the lumbar triangle is formed by the lower part of the lateral margin of the latissimus dorsi (of Petit). The external abdominal oblique muscle and the iliac crest complete the triangle laterally and inferiorly. The floor of this space is made up of the internal abdominal oblique muscle.

Another significant anatomical landmark is the auscultation triangle, which is formed by the latissimus dorsi muscle. This space is bounded by the trapezius (superiorly), the medial border of the scapula (medially), and the latissimus dorsi (inferiorly). When a person folds their arms across their chest and bends their trunk forward, the lower pulmonary lobes become subcutaneous within the auscultation triangle and thus available for auscultation of respiratory sounds with a stethoscope.

Blood Supply

The latissimus dorsi muscle is supplied by the thoracodorsal artery (a continuation of the subscapular artery). It enters the muscle on the costal surface, just medial to the lateral border and a few centimetres from the subscapular artery. The lat muscle is supplied by perforating arteries of the 9th-11th posterior intercostal arteries and the 1st-3rd lumbar arteries, in addition to the thoracodorsal artery.

Functions

The latissimus dorsi muscle performs three functions on the shoulder joint due to the multidirectional alignment of its fibres:

1. It extends the flexed arm effectively.

2. It adducts and rotates the arm internally.

3. When the humerus is fixed against the scapula, it pulls the entire pectoral girdle backwards.

These actions determine the latissimus dorsi muscle's complex functional activity. It is a climbing, rowing, and swimming muscle. The latissimus dorsi has several notable supporting functions:

• If the arms are fixed above the head, it can work in conjunction with the pectoralis major to raise the trunk upwards.

• During humeral movements on the shoulder joint, it stabilises the scapula against the thoracic cage.

• With its humeral attachment fixed in people who use crutches and thus have their humerus as the fixed point during standing, the latissimus dorsi helps to pull the trunk forward. This action also causes the pelvis to lift. This action allows people with paraplegia to move their pelvis and trunk.

• It assists forced expiration by compressing the rib cage (accessory muscle of expiration). For this particular reason, the muscle is particularly strained during coughing attacks (“coughing muscle”).

• By compressing the rib cage, it aids forced expiration (accessory muscle of expiration). As a result, the muscle is especially strained during coughing fits (hence the term "coughing muscle").