Jennifer Chu asked:




Latissimus dorsi is the most powerful muscle of the back and pain and spasm in this muscle can cause significant lower back pain and aggravation of neck pain.

It is a very large, triangular muscle that extends from the arm pit to the lower back. It arises from the lower six thoracic vertebrae, lumbosacral fascia, the crest of the pelvic bone called the iliac bone and also from the lower three to four ribs. It inserts into the floor of the groove on the upper front of the arm bone (humerus) known as the bicipital groove and is very close to the shoulder joint.

Its action is to bring the arm closer to the body (adduction), rolling the arm inward as in placing the hand on the back of the body (internal rotation) and to bring the arm backward (extension). The nerve supply to this muscle is the thoracodorsal nerve which carries the spinal nerve root fibers of the C6, C7 and C8 nerve roots.

This very powerful muscle can lift the body of the ground as in crutch walking, climbing up a pole and chinning the body on the horizontal bar. In these activities, it works together with the pectoralis major and the abdominal muscles. It is used in swimming especially in the down stroke of a crawl and most swimmers have very over developed latissimus dorsi muscles giving them a V-shape with wide shoulders narrowing down to a small waist.

Rowing type activities is primarily the function of the latissimus dorsi muscle. In pitching of baseball, it contracts very strongly in the late cocking phase. Acceleration of the arm as in throwing type activities involve lengthening contraction of the latissimus dorsi muscle which makes this muscle prone to injury.

Overhead activities, especially that against resistance as in climbing are conditions that require lengthening contractions that injure this muscle. Pulling down and chin up activities require tremendous shortening contraction of the latissimus dorsi.

This muscle also raises the lower ribs and assists in respiration. Many patients with pain and spasm of the latissimus dorsi muscle will complain of pain on deep breathing and pain in the sides of the chest wall.

The latissimus dorsi muscle is the bridge between the lower back and the neck. Therefore, the neck and the entire middle and lower back have to be treated in order to treat this muscle properly. Without latissimus dorsi muscle functioning properly, lifting type activities will be performed by shoulder shrugging. Therefore, pain along the slope of the shoulders which is primarily from pain in the trapezius muscles cannot be treated by local treatment only to the trapezius muscle. Pain in the trapezius muscle area will thus recur unless the latissimus dorsi muscle is first treated.

Similarly, low back pain treatment must always involve treating the latissimus dorsi muscle. Because of their insertions into the thoracolumbar fascia, latissimus dorsi muscle can never be properly treated until the gluteus maximus muscle is included in the treatment.

Any symptom of pain and discomfort involving the back must therefore involve the “trio treatment” that involves the gluteus maximus, latissimus dorsi and trapezius muscles.

Since the latissimus dorsi muscle is constantly exposed to lengthening contractions with activities of daily living, most people have chronic tightness and shortening of this muscle. This leads to significant limitation of range of motion of the shoulder for internal rotation and extension.
Therefore, further injury to the commonly injured C6 and C7 nerve roots will further tighten and shorten the latissimus dorsi muscle making it very difficult to treat.