The sciatic nerve, the largest in the body, controls muscles in the back of your knee and lower leg and also provides sensation to the back of the thigh, lower leg and sole of the foot. A problem with this nerve is called sciatica and may lead to pain, numbness, weakness or tingling from the lower back on down. Sciatica usually affects one side of the body. Causes may be a ruptured disc, narrowing of the spinal canal (spinal stenosis) or an injury.