The piriformis is a muscle located near the buttock, and can be prone to getting tight and inflamed.
Why is it an important muscle? The sciatic nerve runs alongside it, and if the muscle is inflammed it can compress the nerve, causing sciatica symptoms. This condition is called piriformis syndrome. The piriformis is also used for walking/running.
It is important to keep this muscle happy in order to maintain a healthy gait, as well as prevent sciatic nerve compression.
Hold this stretch for 30 seconds and perform 2 to 3 on each side. This can be done as little as once per week for those maintaining a healthy muscle or daily for those suffering with an extremely tight piriformis.
Good luck and stay healthy!