The piriformis is a small, pear-shaped muscle buried deep in the buttock. It attaches from the sacrum at the base of the spine to the top of the thigh bone. Its job is to rotate and stabilise the hip, particularly during walking, standing, and changing direction.
The sciatic nerve, which runs from the lower back down the leg, passes very close to (and in some people, directly through) the piriformis. When the muscle becomes tight, overactive, or irritated, it can compress or irritate the sciatic nerve, producing symptoms that mimic sciatica.