Definition: The piriformis is a small, bilateral muscle deep in the buttocks, under the more well-known gluteus maximus. The piriformis attaches to the front side of the sacrum at one end, then passes through the pelvis to insert into the greater trocanter, the knobby part at the top of your femur, or thigh bone. On its way through the pelvis, the piriformis crosses the sciatic nerve. If a tight piriformis places pressure on the nerve, sciatica may result. Doing stretches to lengthen the piriformis can relieve some kinds of sciatic pain. One of the best stretches for the piriformis is this version of pigeon pose.