The piriformis muscle is a deep muscle in the buttock, which primarily rotates the hip. It also flexes the hip (knee to chest), brings the thigh out to the side, and stabilises the pelvis. The reason this muscle is so clinically relevant is that the sciatic nerve interacts closely with it. There are a number of relationships that piriformis and the sciatic nerve might have; for most people the nerve will pass under the muscle, but part or all of the nerve can pass right through the piriformis. This isn’t a problem in itself, but leaves the nerve more vulnerable to irritation, for example, if the muscle gets tight.
Symptoms often include pain in the buttock, particularly when performing actions that activate the muscle, or when sitting. Driving can be particularly painful, as the hip is more flexed than in an office or dining chair, and there’s less opportunity to move. The buttock pain may extend further down the back of the leg, as Piriformis Syndrome is a form of sciatica. In this case, the pain would likely be sharp or shooting in a defined line. Typically the cause is something to do with the muscle, be it a tightness, inflammation, or trauma. Possible reasons for an increase in tone include poor gait to neglecting stretching after exercise, maybe even the trauma on the buttock of long distance cycling. Considering the movements piriformis is responsible for, prolonged sitting will shorten the muscle, allowing it to tighten, so sitting posture can be a predisposing factor, not just an aggravating activity.
It is possible for sciatic pain to come jointly from involvement of the piriformis muscle and other causes, such as an irritated intervertebral disc, but as it is unclear just how many cases of sciatic pain involve the piriformis, it is often overlooked. For most patients with a tight piriformis, I recommend the figure of 4 exercise to complement osteopathic treatment.