Quadratus lumborum

What it is

A flat, roughly four-sided muscle deep in the back of the abdomen, spanning from the back of the pelvic rim up to the twelfth rib and the lumbar vertebrae. It lies behind the abdominal contents, alongside psoas major.

Its position — deep, posterior, between two fixed points — makes it impossible to palpate reliably despite being frequently blamed for lower back symptoms.

What it does

Side-bends the trunk towards its own side, and is the principal muscle for that in the lower back.

It also anchors the twelfth rib during breathing, holding it down so the diaphragm has something to pull against. That is a genuine and often-overlooked job.

Acting on one side with the leg hanging free, it hitches the pelvis upward — which is part of how the leg clears the ground in walking.

Both sides together extend the lumbar spine and stabilise it.

What loads it

Side bending under load, single-sided carries, and any position where the trunk resists being bent sideways. A suitcase carried in one hand loads the opposite side of the trunk continuously.

What is commonly got wrong about it

As the universal explanation for lower back pain. It is a very popular culprit, partly because the region it occupies is where a lot of people feel pain. Since it cannot be palpated reliably through the overlying erector spinae and abdominal wall, the confidence with which it is identified as the source is not well founded. Lower back pain is common, usually not attributable to a single structure, and outside the scope of this site.

That it is a "core" muscle in the fashionable sense. It stabilises the trunk, like several other muscles, and it has no special status.

That stretching it is straightforward. Side bending moves the whole lumbar spine and the abdominal wall with it.