The piriformis muscle is a deep muscle located in the buttock and connects to the sacrum (base of the spine) and the femur (thigh bone). Piriformis syndrome refers to compression of the sciatic nerve as it passes through or under the piriformis muscle. Excessive use of the piriformis muscle without adequate stretching can cause hypertrophy (bulking) of the muscle causing it to compress the sciatic nerve.
Symptoms of piriformis syndrome include deep buttock pain as well as a burning sensation, pins and needles and/or numbness down the back of the thigh. Causes of piriformis syndrome include excessive running, walking, hiking, prolonged sitting, trauma, or due to an anatomy variation (it is more common in people whose sciatic nerve travels through the piriformis muscle, than those whose sciatic nerve travels beneath the piriformis muscle).