The piriformis muscle is a relatively small muscle that originates at the sacral spine and attaches at the top of the femur. The symptoms of piriformis syndrome are generally a tightening or spasm of the muscle. It is often thought to be sciatica since the sciatica nerve runs nearby. Those that suffer with piriformis syndrome complain of a toothache-like pain in the hip that can move into the back of the thigh and even along the outside of the knee. It can also cause pain in the small if the back.
In daily life, piriformis syndrome can be triggered by sitting too much or a sedentary lifestyle. How much do you sit everyday? If your look for piriformis on the internet and actually see the muscle, it is understandable why inactivity can cause problems. Improper footwear or wearing worn out shoes can be problematic as can doing too much (like running) too fast. In any event, it is a good idea to visit your health care provider for a diagnosis if you think this is something you might be dealing with. Don't let it drag on. It can be a nasty little muscle!
There are several good stretches for the piriformis both seated and lying down. While I am tempted to share one with you, it is better and safer for you to actually see it in person. Of course, the best thing to take away from this is prevention. Remember to sit less and sit properly. Good posture means your weight is evenly disturbed when standing without the body weight shifted to one side.
Strengthen the muscles of the hip and thigh and follow up with some good stretching.
Resources: Prescriptive Stretching by Kristian Berg
ACSM's Certified news April-June 2012 Piriformis Syndrome by Jeffery S. Harrison CSCS, NSCA-CPT