Sciatica
Sciatica is pain that starts in the back and radiates down one of the legs. It is caused by pressure on the sciatic nerve which runs down into the leg. The cause of pressure can be varied from a slipped (prolapsed) disk to muscle tension (Piriformis Syndrome) or something less common such as tumors, bony growths and infections.
Depending on where the sciatic nerve is pinched will determine where you feel pain which can radiate to the front of the knee or down the back of the leg to the foot.
Treatment consists of massage to warm and relax muscle tension and stretching techniques to release the nerve.