Päärynänmuotoinen lihas (musculus piriformis) kuuluu takaraajan lihaksiin. Se lähtee ristiluusta sekä koiralla ligamentum sacrotuberalesta ja päättyy reisiluun trochanter majoriin. Sen tehtävänä on lonkkanivelen ojennus, ulkokierto sekä raajan loitontaminen. Sen erikoisuutena on tehtävän vaihtuminen ulkokierrosta sisäkiertoon, kun lonkkanivel on tarpeeksi koukussa.
(source: Wikipedia)
The piriformis muscle and its surroundings: 
As shown in the picture above, the piriformis muscle is part of the external rotators of the hip and some might even consider it a ‘muscle ligament’, similar to the analogy of the rotator cuff muscles and the shoulder joint. It is a commonly underlooked muscle, one that can be responsible for sciatica pain: “piriformis syndrome”. The sciatic nerve can run in three different directions: under the muscle, over it and last but most unfortunate in the muscle itself.
People usually feel some pain radiating down the leg (at the back) all the way to the last toe (in the most severe cases). Some numbness can also be experienced around the butt cheek area and close to the posterior iliac spine (that’s the bony landmark located between the spine and the pelvis). One might consider a bulging disk applying pressure on the sciatic nerve and this has to be ruled out before engaging in any strengthening/stretching plan.