Serratus posterior muscles
The serratus posterior muscles are two thin, intermediate back muscles which lie above the intrinsic back musculature. Namely, the serratus posterior muscles are:
- The serratus posterior superior muscle
- The serratus posterior inferior muscle
The functions of the serratus posterior muscles include elevation of the ribs (serratus posterior superior) and depression of the ribs (serratus posterior inferior)
|Serratus posterior superior||
Origin: Nuchal ligament, spinous processes of vertebrae C7-T3
Insertion: Superior borders of ribs 2-5
Innervation: 2nd-5th Intercostal nerves
Function: Elevates ribs
|Serratus posterior inferior||
Origin: Spinous processes of vertebrae T11-L2
Insertion: Inferior borders of ribs 9-12
Innervation: Anterior rami of spinal nerves T9-T12 (a.k.a. 9th-11th Intercostal nerves + subcostal nerve)
Function: Depresses ribs/ Draws ribs inferoposteriorly
This article will discuss the anatomy of the serratus posterior muscles.
Anatomy and innervation
Both originate from the spinous processes and attach on the ribs, which is why they are also referred to as spinocostal muscles. There are two of them, as follows:
- Serratus posterior superior muscle: runs from nuchal ligament and spinous processes of vertebrae C7-T3. This muscle is covered by the rhomboids and the trapezius.
- Serratus posterior inferior muscle: originates from the spinous processes of vertebrae T11-L2. From there, it ascends craniolaterally to the 9th to 12th ribs. The latissimus dorsi lies above this muscle.
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Both muscles are counted among the secondary back muscles, meaning that they migrated to the back in the course of embryogenesis. They are supplied by the intercostal nerves deriving from the anterior rami of the spinal nerves.
The serratus posterior muscles contribute to the movement and stabilization of the vertebral column and the thorax. The serratus posterior superior elevates the ribs and thus supports the inspiration (accessory muscle of inspiration). Furthermore, the activation on both sides extends the thoracic vertebral column while a unilateral contraction rotates it to the opposite side.
In contrast the serratus posterior inferior helps during expiration by depressing the ribs (accessory muscle of expiration). This muscle can also extend (bilateral contraction) and rotate (unilateral contraction) the spine.
The disproportionate use of the serratus posterior superior and other back muscles (e.g. cradling the phone between ear and shoulder) can result in a condition known as the scapulocostal syndrome. It is characterized by pain and paresthesia along the medial border of scapula radiating to the neck, chest and upper extremities. Particularly, pain felt in the little finger during palpation of the muscle is one of the classic signs.
Clinically, the scapulocostal syndrome is easily mistaken for a lesion of the spinal nerves of the cervical vertebral column (cervical radiculopathy) which typically presents with numbness, muscle weakness and further neurological deficits (e.g. weak or absent reflexes). As differential diagnosis, arthritis or rotator cuff rupture should also be considered.