Serratus Posterior Muscles: Location and Action

The Serratus posterior muscles consist of a superior and inferior (upper and lower) which are located on each side of the upper and lower part of the thoracic region of the back. They are thin quadrilateral muscles which are deep to the true back muscles such as the rhomboids, and in the case of the serratus posterior superior, the muscles of the upper arm. These muscles derive their name from their shape, the word serratus meaning serrated or saw-toothed. They bear no functional relationship to the more well-known serratus anterior muscles.

Serratus Posterior Superior Muscle
Serratus Posterior Superior Muscle

The position and shape of the serratus posterior superior muscles could easily cause them to be confused with the rhomboid muscles. Although they do attach to the spine, they do not attach to the scapula, as do the rhomboids, but to the ribs, passing underneath the shoulder blades. These muscles, then, do not serve as back muscles or upper limb muscles, but rather as muscles of inspiration. Specifically, the serratus posterior superior is thought to elevate the second through fourth ribs and the serratus posterior inferior is thought to pull on the lower ninth through twelfth ribs. In this way, they both help with breathing by enlarging the thoracic cavity, presumably with assistance from the scalenes and the intercostals of the ribs. Evidence from electromyographic studies, however, do not support this respiratory function of the serratus posterior inferior. It may contribute to rotation of the spine unilaterally, and to extension of the spine, bilaterally.1Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. ‘Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 329. Print.,2Jenkins, David B. Hollinshead’s Functional Anatomy of the Limbs and Back. Saunders/Elsevier, 2009.,3Cramer, Gregory D., and Susan A. Darby. Clinical Anatomy of the Spine, Spinal Cord, and ANS. Mosby, 2017.

Serratus Posterior Inferior Muscle
Serratus Posterior Inferior Muscle

Serratus Posterior Origin, Insertion, and Actions

Superior Origin: Legamentum nuchae; spinous processes of the seventh cervical and upper two or three thoracic vertebrae.

Inferior Origin: Spinous processes of the lower two thoracic and upper two lumbar vertebrae

Superior Insertion: Upper ribs, second through fifth (with some variations)

Inferior Origin: Lower three to four ribs

Superior Action: Elevations of second through fourth ribs, thus assisting in inspiration by enlarging thoracic cavity.

Inferior Action: [Possibly ] pulls lower ribs inferiorly (downward), and helps stabilize them against the upward pull of the diaphragm, thus assisting in exhalation; possible rotation when contracted unilaterally and extension of the spine when contracted bilaterally.

Sources   [ + ]

1. Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. ‘Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 329. Print.
2. Jenkins, David B. Hollinshead’s Functional Anatomy of the Limbs and Back. Saunders/Elsevier, 2009.
3. Cramer, Gregory D., and Susan A. Darby. Clinical Anatomy of the Spine, Spinal Cord, and ANS. Mosby, 2017.