Updated December 16, 2014.
The piriformis muscle is a deep muscle located beneath the gluteal (butt) muscles. The piriformis muscle laterally rotates and stabilizes the hip. This muscle is important for athletes who participate in running sports that require sudden changes of direction. The piriformis works along with other hip rotators to turn the hips and upper leg outward (external rotation of the hip). Strong and flexible hip rotators keep hip and knee joints properly aligned during activity and help prevent sudden twisting of the knee during quick side-to-side movements, quick turns, lunges or squats.
A condition called "piriformis syndrome," which causes pain deep in the hip and buttock, is believed to be caused when the piriformis muscle compresses the sciatic nerve.

Stretching and strengthening a tight or weak piriformis muscle has been found to reduce or alleviate this pain in some athletes.

How to Stretch the Piriformis Muscle

Simple Piriformis Muscle Stretch Options

• Sitting Cross-Legged. One of the easiest ways to keep your hips open and stretch your piriformis muscle is by sitting cross-legged on the floor for several minutes a day.
• Piriformis Chair Stretch. Another easy way to stretch out the piriformis, especially if you have a desk job, is to cross one leg over the other with your ankle resting on the knee of the opposite leg. Gently press down on the inside of the knee and slowly lean forward until you feel a mild stretch in the hips.
• Lying Piriformis Stretch. A more intense stretch can be performed while laying on your back on the floor. Cross the right leg over the left, with the right ankle resting on the left knee. Slowly lift the left foot off the floor and toward you while you apply gentle pressure to the inside of the right knee. Hold 20 to 30 seconds, and repeat on the other side.

Advanced Piriformis Stretch - Pigeon Pose

This stretch, pictured above, is a more advanced piriformis and hip stretch, in which you use your whole body weight to stretch the piriformis, the IT band and other hip rotators. Use caution as you get in to and out of this pose.

• Start in a push-up position on your hand and toes.
• Slide your right knee forward toward your right hand. Angle your knee, so the outer ankle is touching the floor (see picture).
• Slide your left leg back as far as comfortable.
• Keep your hips square to the floor.
• You should feel a deep stretch in your right glutes (buttock), hip and the outer thigh.
• You can either stay up on your hands or fold forward and let your forearms rest on the floor in front of you or fully extend your arm in front of you.
• Breathe slowly and deeply from your belly. Hold the stretch 30 seconds to 60 seconds and release. Repeat on the other leg.