Weak hip rotators, which lie under the gluteal muscles, can put strain on the piriformis muscle, causing it to place irritating pressure on the sciatic nerve.
http://www.runnersworld.com/article/printer_friendly/0,5046,s6-78-81-0-9107,00.html
http://www.runnersworld.com/article/printer_friendly/0,5046,s6-78-81-0-9107,00.html