Piriformis

What it is

A small, pear-shaped muscle deep in the buttock — pirum is Latin for pear. It runs from the front of the sacrum, passes out through the greater sciatic foramen, and attaches to the top of the greater trochanter.

It is one of a group usually called the deep external rotators, along with the gemelli, the obturators and quadratus femoris. All of them are small, all sit under gluteus maximus, and none of them can be felt or trained individually.

Its fame comes entirely from its neighbour. The sciatic nerve emerges from the pelvis in the same region, usually passing beneath the muscle, though the anatomy varies between people and in a minority the nerve passes through or above it.

What it does

Rotates the thigh outward. When the hip is already flexed to around ninety degrees its line of pull changes and it contributes to abduction instead — a genuinely unusual property, and one reason its role is described inconsistently.

In walking it helps control the rotation of the pelvis over the planted leg.

What loads it

Nothing specifically or exclusively. It works as part of the deep rotator group during walking, changes of direction, and any movement where the hip rotates under load.

Exercises marketed as piriformis exercises are loading the whole deep rotator group, and usually gluteus maximus along with it.

What is commonly got wrong about it

"Piriformis syndrome" as a confident diagnosis. The idea is that this muscle compresses the sciatic nerve and produces buttock and leg pain. The condition is real as a described entity, but it is genuinely contested — there is no agreed diagnostic test, the symptoms overlap heavily with problems originating in the lumbar spine, and estimates of how common it is vary enormously depending on who is counting. It is diagnosed online far more often than it is diagnosed carefully.

That stretching it fixes sciatic pain. Leg pain that travels has several possible sources and most of them are not this muscle. This is exactly the situation the site will not help with, and where an examination is the only thing that can distinguish the possibilities.

That you can feel it working. You cannot separate it by sensation from the other deep rotators or from the gluteal muscles lying over it.