Longissimus Thoracis (Lumbar Part)

Back · Trunk

The Longissimus Thoracis (Lumbar Part) is a Trunk muscle within the Back group. It arises from Accessory and transverse processes of the lumbar vertebrae and attaches to Medial part of the iliac crest via the lumbar intermuscular aponeurosis. It is responsible for several movements: Bilaterally extends the lumbar spine; unilaterally laterally flexes it. Innervation is provided by the Posterior (dorsal) rami of lumbar spinal nerves.

Origin, Insertion, Action & Nerve

Origin

Accessory and transverse processes of the lumbar vertebrae

Insertion

Medial part of the iliac crest via the lumbar intermuscular aponeurosis

Action

Bilaterally extends the lumbar spine; unilaterally laterally flexes it

Nerve

Posterior (dorsal) rami of lumbar spinal nerves

Attachments explained

The Longissimus Thoracis (Lumbar Part) 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

Accessory and transverse processes of the lumbar vertebrae

Insertion

Medial part of the iliac crest via the lumbar intermuscular aponeurosis

Action & function

When the Longissimus Thoracis (Lumbar Part) 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 Longissimus Thoracis (Lumbar Part) 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 Longissimus Thoracis (Lumbar Part), 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 Longissimus Thoracis (Lumbar Part) (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Longissimus Thoracis (Lumbar Part), 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 Longissimus Thoracis (Lumbar Part) 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.