The piriformis muscle is a very important muscle that lies deep in your buttocks. Oftentimes, runners are affected by stress and injury to this muscle, but it can definitely be a problem for everyone. As seen in the picture above, it sits just on top of the sciatic nerve. In approximately 15% of people, the sciatic nerve actually travels right through the muscle!!! Could you imagine??? Anytime that muscle becomes inflamed or injured, pressure can be applied on the nerve causing a host of issues.
The main function of the piriformis muscle is the lateral (external) rotation of the hip when the hip is extended. Think of it this way…when standing, the piriformis acts to rotate your leg so that the toe points away from the body. Other lateral rotators of the hip include the superior gemellus , obturator externus, obturator internus, inferior gemellus and quadratus femoris. When learning these muscles in Anatomy class, we used this pneumonic…”please send out information quickly”. P for piriformis; S for superior gemellus; O for both obturators; I for inferior gemellus; and Q for quadratus femoris.
When the hip is flexed, however, the piriformis has a different action. It actually abducts the femur (thigh bone) meaning it brings the femur away from the body. Abduction of the thigh with hip flexion is a very important movement during the gait cycle when walking by shifting the bodyweight to the opposite side of the foot that is lifted which prevents falling.
Injury to the piriformis muscle can lead to trigger point formation. When trigger points are formed, pain manifests. The figure above demonstrates referral patterns of pain when the trigger points are active. Also, depending on the severity of the injury, an inflamed piriformis muscle can apply pressure to the sciatic nerve causing sciatica symptoms to occur. This is called Piriformis Syndrome. As seen in the figure, irritation of the piriformis can refer pain to the sacroiliac region of the low back which can make a diagnosis of the syndrome difficult. If the sciatic nerve is involved, you will experience pain down the back of the leg into the calf and maybe even all the way into the foot. If the syndrome goes untreated, numbness, tingling, and weakness may happen as well.
Due to the action of the muscle itself, symptoms worsen with prolonged sitting and with activity…such as running, especially uphill. There are a wide variety of risk factors that may increase the likelihood of developing piriformis syndrome:
sedentary jobs where you sit most of the day, especially in a chair where your knees sit higher than your hips