Iliocostalis Lumborum

Origin, Insertion, Action & Nerve

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.

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)

Anchor iliocostalis lumborum as the lowest and most lateral erector spinae column, spanning from the sacrum and iliac crest up to the lower ribs. Reason from that vertical, lateral position: pulling equally on both sides extends and straightens the lower back, while pulling on one side bends the trunk to that side, exactly the demands of bending and lifting.

Distinguish it from quadratus lumborum, which lies deeper and also laterally flexes the trunk, by remembering that iliocostalis is part of the superficial erector spinae supplied by dorsal rami. Linking it in a chain with iliocostalis thoracis and cervicis above shows how the column relays force from the pelvis all the way up the back.

Frequently asked questions

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