The piriformis is a small muscle located deep in the buttock, behind the gluteus maximus. It runs diagonally from the lower spine to the upper surface of the femur, with the sciatic nerve running underneath or through the muscle. The piriformis muscle helps the hip rotate, turning the leg and foot outward.
As a result of overuse, injury, or strain, the piriformis muscle can tighten, swell, or spasm. Sports that involve repetitive forward movement of the legs, or problems in surrounding joints like the sacroiliac joints are typical causes of piriformis syndrome.
The location of the piriformis muscle near the sciatic nerve means that when the piriformis muscle is irritated or injured, it can affect the sciatic nerve as well.