A scan of Riker's leg, highlighting the sciatic nerve
The sciatic nerve is the one of the largest nerves in the Human body, running from the pelvis to the back of the knee.
In 2365, Commander Riker was infected with Surata microbes, which invaded his nervous system starting from his sciatic nerve. The contagion was neutralized only with unconventional neural therapy by Dr. Pulaski and Counselor Troi. (TNG: "Shades of Gray")
In 2372, Samantha Wildman suffered lower back pain because her unborn child was putting pressure on her sciatic nerve. (VOY: "Tattoo")