The piriformis stretch or the pigeon pose from yoga is a common therapeutic “stretch” that many fitness professionals, physiotherapists, and chiropractors recommend for a tight piriformis. It is even recommended by world renowned leaders in these industries. Although the pigeon stretch may be beneficial or enjoyable, it does not stretch the piriformis and it never will.
To understand why the pigeon pose does not stretch the piriformis, you must first understand the basic concept of shortening/contracting vs lengthening/stretching a muscle. For example, to contract the bicep brachii, the arm would bend at the elbow due to the shortening of the muscle fibers. It also supinates and flexes the shoulder but this is not relevant to illustrate this concept. In order to stretch a muscle it must be lengthened or elongated along its fibers. Therefore, you must extend or straighten your elbow to elicit a stretch for the bicep brachii. You must do the opposite action of a muscle to elongate it.
The piriformis mainly externally rotates the hip but it also abducts the hip when it is flexed. The pigeon pose, or even the supine figure four stretch, shortens the piriformis since it mimics the concentric action of the muscle. The muscles that are being lengthened during these stretches are the internal rotators of the hip, which consist of the tensor fascia lata, anterior fibers of the gluteus medius, and the gluteus minimus.
As you can see, in order to stretch the pirifromis you must do the opposite function of the muscle. Below Dr. Aaron Horschig, founder of Squat University, demonstrates an effective method of stretching your piriformis along with the other external rotators of the hips.
Dr. Brandon Parnham, DC 