Serratus Posterior Inferior
Thorax · Trunk
The serratus posterior inferior is a thin, quadrilateral muscle of the intermediate layer of the back, lying deep to the latissimus dorsi at the thoracolumbar junction. It arises from the spinous processes of the eleventh thoracic to the second or third lumbar vertebrae and their supraspinous ligaments, and its fibres run upward and laterally to insert onto the inferior borders of the ninth to twelfth ribs, lateral to their angles. Traditionally described as depressing and stabilising these lower ribs, it is often considered to steady them against the upward pull of the diaphragm during respiration. It is supplied by the anterior rami of T9 to T12, the intercostal and subcostal nerves.
Origin, Insertion, Action & Nerve
Origin
Spinous processes of T11–L2/L3 and the supraspinous ligaments
Insertion
Inferior borders of ribs 9–12, lateral to their angles
Action
Depresses and stabilizes the lower ribs (9–12)
Nerve
Anterior rami of T9–T12 (intercostal and subcostal nerves)
Attachments explained
The Serratus Posterior Inferior is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Spinous processes of T11–L2/L3 and the supraspinous ligaments
Insertion
Inferior borders of ribs 9–12, lateral to their angles
Action & function
When the Serratus Posterior Inferior contracts, it produces the following movement: Depresses and stabilizes the lower ribs (9–12).
As part of the Thorax group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Serratus Posterior Inferior receives its nerve supply from the Anterior rami of T9–T12 (intercostal and subcostal nerves). This nerve carries fibres from spinal segment(s) T9, T12.
Because a muscle can only contract when its nerve is intact, injury to the Anterior rami of T9–T12 or to its spinal roots (T9, T12) can weaken or paralyse the Serratus Posterior Inferior, impairing the movements it normally produces (depresses and stabilizes the lower ribs (9–12)). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Serratus Posterior Inferior (the OIANS method)
Study serratus posterior inferior as one of the two intermediate-layer respiratory muscles of the back, paired with serratus posterior superior at the opposite end of the thorax. Its low position and upward-lateral fibre direction explain its action of drawing the lower ribs downward and fixing them, which is easiest to recall by noting that it sits at the thoracolumbar junction where the diaphragm attaches nearby.
Use its innervation to separate it from the deep back muscles: unlike the levatores costarum, this respiratory muscle is supplied by anterior rami, specifically the intercostal and subcostal nerves of T9 to T12. Matching the nerve levels to the ribs it moves, 9 through 12, lets you recover both the attachments and the nerve together.
Frequently asked questions
Where is the Serratus Posterior Inferior located?
The Serratus Posterior Inferior is a muscle of the Thorax group, located in the Trunk.
What is the origin of the Serratus Posterior Inferior?
Spinous processes of T11–L2/L3 and the supraspinous ligaments
What is the insertion of the Serratus Posterior Inferior?
Inferior borders of ribs 9–12, lateral to their angles
What movements does the Serratus Posterior Inferior produce?
Depresses and stabilizes the lower ribs (9–12)
What nerve supplies the Serratus Posterior Inferior?
Anterior rami of T9–T12 (intercostal and subcostal nerves)
Is the Serratus Posterior Inferior free to study in OIANS?
The Serratus Posterior Inferior is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Trunk is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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