Latissimus dorsi muscle

Origin and insertion

The latissimus dorsi muscle spans over the lumbar and lower thoracic regions of the back. Depending on the origin, one can distinguish four parts of this muscle:

• Vertebral part: originating from the spinous processes of the 7th to 12th thoracic vertebrae and the thoracolumbar fascia
• Costal part: having origins from the 9th to 12th ribs
• Iliac part: starting from the iliac crest
• Scapular part: starting from the inferior angle of the scapula (inconstant)

All the fibers converge towards the proximal humerus. During their course, the upper vertebral and scapular fibers take a nearly horizontal course, the lower vertebral and iliac oblique, and the costal almost vertical. At the level of the teres major muscle, the fibers turn spirally around it, with the lower part of the latissimus dorsi inserting proximally at the humerus and at the upper part more distally. All fibers together attach to the floor of the intertubercular sulcus of the humerus between the pectoralis major and teres major. 

An easy way to remember the relation of the latissimus dorsi, pectoralis major and teres major muscles as they insert in the intertubercular sulcus is to use the following mnemonic: 'Lady between two majors':

• Lady: Latissimus dorsi
• Majors: Teres major, pectoralis major

Quiz yourself on the anatomy of the superficial back muscles with multiple difficulty levels.

Relations

The latissimus dorsi muscle is found superficially in the lower two-thirds of the trunk, covering the serratus posterior muscles. The teres major muscle lies superior to latissimus dorsi. The adjoining fibers of these two muscles are united, but separated by a bursa towards their humeral attachments. As they span the interval between the scapula and proximal humerus, the latissimus dorsi and teres major form the posterior axillary fold. The fold is accentuated when the arm is adducted against resistance. During this movement, it is possible to trace the whole inferolateral border of latissimus dorsi to its attachment to the iliac crest. 

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

The latissimus dorsi muscle comprises another important anatomical landmark; the auscultation triangle. The borders of this space are the trapezius (superiorly), medial border of scapula (medially), and latissimus dorsi (inferiorly). When a person folds their arms across the chest and bends the trunk forwards, the lower pulmonary lobes become subcutaneous within the auscultation triangle, and thus available for auscultation of the respiratory sounds with a stethoscope.

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Innervation

The innervation is carried by the thoracodorsal nerve (C6-C8), a branch of the brachial plexus.

Blood supply

The thoracodorsal artery (a continuation of the subscapular artery), supplies the latissimus dorsi muscle. It enters the muscle on its costal surface, medial to the lateral border and a few centimetres from the subscapular artery. 

As well as the thoracodorsal artery, perforating arteries of the 9th-11th posterior intercostal arteries, and 1st-3rd lumbar arteries supply the lat muscle.

Functions

Due to the multidirectional alignments of its fibers, the latissimus dorsi muscle exhibits three actions on the shoulder joint:

• It effectively extends the flexed arm
  
• It adducts and internally rotates the arm
  
• If the humerus is fixed against the scapula, it pulls the pectoral girdle backward as a whole
  

These actions determine the complex functional activity of the latissimus dorsi muscle; it is a climbing muscle, rowing muscle, and a swimming muscle. 

There are several noteworthy supporting functions of the latissimus dorsi:

• If the arms are fixed above the head, it can act in synergy with pectoralis major to elevate the trunk upwards
• It stabilizes the scapula against the thoracic cage during humeral movements on the shoulder joint
• In people who use crutches and therefore have their humerus as the fixed point during standing, the latissimus dorsi helps to pull the trunk forwards with its humeral attachment being fixed. 
  • This action also causes the lifting of the pelvis. In people with paraplegia, this action enables the movements of the pelvis and trunk.
• It assists forced expiration by compressing of the rib cage (accessory muscle of expiration). For this reason, the muscle is particularly strained during coughing attacks (“coughing muscle”).