The piriformis muscle is infamous for its idiopathic "tightness" causing all kinds of buttaches. However, the real cause of its tightness is actually weakness. When weak, it becomes abrasive, rubbing down on the sciatic nerve. Its weakness may also cause posterior hip impingement, pudendal neuralgia (in cooperation with the obturator internus), and more. The piriformis is also unique (amongst the deep six hip rotators) in its role of sacroiliac stability, assisting the sacrospinous ligament by pulling the sacrum down, thus increasing force closure.

Strengthening the piriformis is essential in order to increase its work capacity, so that it can stabilize the hip without becoming inflamed.

In relation to this, I have come to the conclusion, however, after an immense amount of trial and error, that the piriformis muscle is not a main lateral hip rotator, and thus can not optimally be exercised as such.

Because the piriformis inserts to the part of the greater trochanter, its role is much more similar to the gluteus medius than the more inferior deep six muscles, such as the obturator externus, which has a more direct role in external rotation. Exercising the piriformis by pulling the superior part of the greater trochanter toward the inferolateral sacrum, has been shown to be very effective.