Iliocostalis Lumborum

Back · Trunk

The Iliocostalis Lumborum is a Trunk muscle within the Back group. Originating at Medial sacral crest, medial iliac crest, and thoracolumbar fascia, it inserts at Inferior borders of the angles of the lower six or seven ribs. It is responsible for several movements: Bilaterally extends the lumbar spine; unilaterally laterally flexes it. Its nerve supply is the Posterior (dorsal) rami of lumbar spinal nerves.

Origin, Insertion, Action & Nerve

Origin

Medial sacral crest, medial iliac crest, and thoracolumbar fascia

Insertion

Inferior borders of the angles of the lower six or seven ribs

Action

Bilaterally extends the lumbar spine; unilaterally laterally flexes it

Nerve

Posterior (dorsal) rami of lumbar spinal nerves

Attachments explained

The Iliocostalis Lumborum 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

Medial sacral crest, medial iliac crest, and thoracolumbar fascia

Insertion

Inferior borders of the angles of the lower six or seven ribs

Action & function

When the Iliocostalis Lumborum contracts, it produces the following movements: Bilaterally extends the lumbar 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 Lumborum receives its nerve supply from the Posterior (dorsal) rami of lumbar spinal nerves.

Because a muscle can only contract when its nerve is intact, injury to the Posterior can weaken or paralyse the Iliocostalis Lumborum, impairing the movements it normally produces (bilaterally extends the lumbar spine). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Iliocostalis Lumborum (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Iliocostalis Lumborum, 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 Lumborum 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.