Iliocostalis Thoracis
Back · Trunk
The iliocostalis thoracis is the thoracic portion of the iliocostalis column, the most lateral subdivision of the erector spinae, bridging the lumbar and cervical parts. It arises from the angles of the lower six ribs, from ribs 7 to 12, and ascends to insert on the angles of the upper six ribs and onto the transverse process of C7. Acting on both sides it extends the thoracic spine, and acting on one side it laterally flexes it toward that side; by connecting rib angle to rib angle it also helps stabilise the thoracic cage. It is supplied by the posterior (dorsal) rami of the thoracic spinal nerves.
Origin, Insertion, Action & Nerve
Origin
Angles of the lower six ribs (7–12)
Insertion
Angles of the upper six ribs and the transverse process of C7
Action
Bilaterally extends the thoracic spine; unilaterally laterally flexes it
Nerve
Posterior (dorsal) rami of thoracic spinal nerves
Attachments explained
The Iliocostalis Thoracis 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
Angles of the lower six ribs (7–12)
Insertion
Angles of the upper six ribs and the transverse process of C7
Action & function
When the Iliocostalis Thoracis contracts, it produces the following movements: Bilaterally extends the thoracic spine; unilaterally laterally flexes it.
As part of the Back 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 Iliocostalis Thoracis receives its nerve supply from the Posterior (dorsal) rami of thoracic spinal nerves.
Because a muscle can only contract when its nerve is intact, injury to the Posterior can weaken or paralyse the Iliocostalis Thoracis, impairing the movements it normally produces (bilaterally extends the thoracic spine). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Iliocostalis Thoracis (the OIANS method)
Understand iliocostalis thoracis as the middle link of the iliocostalis chain, taking origin from the lower rib angles and reaching up to the upper rib angles and C7. Because it runs rib angle to rib angle just lateral to the spine, its two-sided contraction extends the thoracic column and its one-sided contraction bends it to that side.
Fix its place by sequence and supply: it sits between iliocostalis lumborum below and iliocostalis cervicis above, so the three parts form a continuous lateral column from pelvis to neck. Remembering that all three share segmental dorsal rami, here the thoracic spinal nerves, reinforces that the entire erector spinae is innervated level by level rather than by one nerve.
Frequently asked questions
Where is the Iliocostalis Thoracis located?
The Iliocostalis Thoracis is a muscle of the Back group, located in the Trunk.
What is the origin of the Iliocostalis Thoracis?
Angles of the lower six ribs (7–12)
What is the insertion of the Iliocostalis Thoracis?
Angles of the upper six ribs and the transverse process of C7
What movements does the Iliocostalis Thoracis produce?
Bilaterally extends the thoracic spine; unilaterally laterally flexes it
What nerve supplies the Iliocostalis Thoracis?
Posterior (dorsal) rami of thoracic spinal nerves
Is the Iliocostalis Thoracis free to study in OIANS?
The Iliocostalis Thoracis 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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