The piriformis muscle is a small, flat muscle which sits deep in the gluteal region (buttocks). The piriformis muscle originates on the lower aspect of the spine (sacrum) and inserts on the top of the femur (thigh bone). The function of the piriformis muscle is to extend and externally rotate the hip (rotate the hip out and extend the thigh back). The sciatic nerve courses deep to the piriformis and in about 15% of the population, the sciatic nerve courses through the piriformis muscle. When the piriformis muscle is tight, shortens or spasms due to faulty mechanics, overuse or injury, it compresses and irritates the sciatic nerve. This results in pain deep in the buttock region and radiating pain down the back of the leg, commonly known as “sciatica.”
Pronation is a complex motion of the foot in which the foot moves up and away from the center of the body. The image to the right illustrates the motion of pronation when weight is not on the foot. Pronation is a normal motion during walking and occurs after the heel contacts the ground. Once the heel contacts the ground, the foot rolls inward, absorbing shock and balancing the foot. When the foot overpronates, the heel and ankle continue to roll inward, resulting in arch collapse, instability and inefficient push off. As the foot and ankle roll in, the leg and knee rotate inward (internal leg rotation). The inward rotation also causes the hip to rotate inward, placing excess tension on the piriformis muscle and compressing the sciatic nerve .
For those with piriformis syndrome, an adequate foot and ankle evaluation should be done. Controlling pronation can be a key factor in minimizing the tension on the piriformis muscle and decreasing the pressure on the sciatic nerve.