Trunk (thorax, abdomen, back)

Serratus Posterior Inferior

Musculus serratus posterior inferior

Origin

Spinous processes of T11-L2 and the thoracolumbar fascia

Insertion

Lower borders of ribs 9 through 12

Function

Pulls the lower ribs downward and fixes them during breathing; assists expiration

Innervation

Intercostal nerves T9-T12

Movement & chain

Acts as an antagonist of the diaphragm by stabilizing the lower ribs against the pull of the diaphragm during deep inspiration.

Palpation

Lies deep to latissimus dorsi at T11–L2 level; palpated as deep muscle tone over ribs IX–XII during forced expiration.

Clinical

Depresses lower ribs during forced expiration and stabilizes diaphragmatic attachments; implicated in lower thoracic flank pain.

Segmental innervation

Intercostal nerves T9-T12.

In clinical practice

Ergonomic patient repositioning (log-roll technique in spinal trauma) and pressure offloading; maintain neutral spinal alignment in bedridden patients.

Region in the model

Trunk (thorax, abdomen, back)