Iliocostalis Thoracis

Back · Trunk

Iliocostalis Thoracis belongs to the Back group of the Trunk. It arises from Angles of the lower six ribs (7–12) and attaches to Angles of the upper six ribs and the transverse process of C7. It is responsible for several movements: Bilaterally extends the thoracic spine; unilaterally laterally flexes it. Its nerve supply is the Posterior (dorsal) rami of 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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Iliocostalis Thoracis, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Iliocostalis Thoracis fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.