Serratus Posterior Inferior

What is the Serratus Posterior Inferior

The serratus posterior inferior, or SPI, is a thin, broad, v-shaped, extrinsic muscle located in the lower back. It belongs to the intermediate layer of back muscles, with the serratus posterior superior being the only other muscle in the group. The muscle originates from the vertebral column and is inserted into the ribcage; hence, it is considered a chest muscle. The name comes from its characteristic serrated appearance. The muscle helps with moving the ribcage during respiration.

Anatomy

Serratus posterior inferior is located where the thoracic and lumbar regions of the spines meet.

Location and Attachments

Origin	Spinous processes of T11 to L2 vertebrae
Insertion	Lower borders of the 9th to 12th ribs

Origin

The muscle arises from the supraspinous ligaments and the spinous processes of the 11th thoracic vertebrae through the 2nd lumbar vertebrae (T11-L2).

Insertion

The muscle fibers then run laterally upward toward the ribs and insert into the lower back borders and outer surfaces of the 9th to 12th ribs. As the muscle fibers course toward their points of insertion, they maintain a relatively consistent thickness and divide into four distinct, flat, fleshy projections, each attaching to one of the ribs. The exact insertion point is slightly lateral to the costal angle of each rib.

Relations With Surrounding Muscles and Structures

The erector spinae muscles and the thoracolumbar fascia lie deep to the serratus posterior inferior, while it itself lies deep to the lower back muscle,  latissimus dorsi. 

The lateral branches of the thoracic spinal nerves from T7 to T12 pierce the muscle on their way down.

Function

Action

Depressing the lower ribs to aid in exhalation

Though there is some debate over the exact function of the muscle, it is known to help with respiration by pulling down the 9th to 12th ribs during expiration, especially in forced exhalation. This action reduces the volume of the thoracic cavity, which causes air to rush out of the lungs.

Research also suggests the muscle’s involvement in proprioception, the ability of the body to sense its own movements and position.

Innervation

Nerve

T9 to T11 intercostal nerves and the subcostal nerve

The muscle is innervated by the intercostal nerves T9 to T11, which originate from the anterior rami of the T9 to T11 thoracic spinal nerves. Additionally, the subcostal nerve running along the inferior border of the 12th rib also provides innervation to the serratus posterior inferior.

Blood Supply

Artery

Subcostal artery and lumbar arteries

Blood supply comes from the subcostal artery, the lowest (12th) posterior intercostal artery that runs along the lower border of the 12th rib. The first 2 lumbar arteries also contribute to the muscle’s vasculature.