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Piriformis syndrome is a muscular disorder that can compress the sciatic nerve causing radicular symptoms into the buttock and along the course of the sciatic nerve.
When the piriformis muscle shortens or spasms due to trauma or overuse, it can compress or strangle the sciatic nerve beneath the muscle. Generally, conditions of this type are referred to as nerve entrapment or as entrapment .
Inactive gluteal muscles also facilitate development of the syndrome. These are important in both hip extension and in aiding the piriformis in external rotation of the femur. A major cause for inactive gluteals is unwanted reciprocal inhibition from overactive hip flexors (psoas major, iliacus, and rectus femoris). This imbalance usually occurs where the hip flexors have been trained to be too short and tight, such as when someone sits with hips flexed, as in sitting all day at work. This deprives the gluteals of activation, and the synergists to the gluteals (hamstrings, adductor magnus, and piriformis) then have to perform extra roles they have not evolved to do. Resulting hypertrophy of the piriformis then produces the typical symptoms.
Overuse injury resulting in piriformis syndrome can result from activities performed in the sitting position that involves strenuous use of the legs as in rowing/sculling and bicycling.
Runners, bicyclists and other athletes engaging in forward-moving activities are particularly susceptible to developing piriformis syndrome if they do not engage in lateral stretching and strengthening exercises. When not balanced by lateral movement of the legs, repeated forward movements can lead to disproportionately weak hip abductors and tight adductors
Another cause for piriformis syndrome is stiffness, or hypomobility, of the sacroiliac joints. The resulting compensatory changes in gait would then result in shearing of one of the origins of the piriformis, and possibly some of the gluteal muscles as well, resulting not only in piriformis malfunction but in other low back pain syndromes as well.
Piriformis syndrome can also be caused by overpronation of the foot.[citation needed] When a foot overpronates it causes the knee to turn medially, causing the piriformis to activate to prevent over-rotating the knee. This causes the piriformis to become overused and therefore tight, eventually leading to piriformis syndrome.