Serratus posterior muscles
Anatomy & Innervation
The serratus posterior muscles are two thin, superficial back muscles which lie above the intrinsic back musculature. 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 the spinous processes of the C6 to T2 vertebrae caudolaterally to the 2nd to 5th ribs. This muscle is covered by the rhomboids and the trapezius.
- Serratus posterior inferior muscle: originates from the thoracolumbar fascia as well as the spinous processes of the lower thoracic vertebrae and upper lumbar vertebrae. From there, it ascends craniolaterally to the 9th to 12th ribs. The latissimus dorsi lies above this muscle.
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.