Sciatica is a symptom of a problem with the sciatic nerve, the largest nerve in the body. It controls muscles in the back of your knee and lower leg and provides feeling to the back of your thigh, part of your lower leg, and the sole of your foot. When you have sciatica, you have pain, weakness, numbness, or tingling. It can start in the lower back and extend down your leg to your calf, foot, or even your toes. It's usually on only one side of your body.