Patella twitch. Artwork of the muscles of the knee from the front, with red arrows showing the direction of movement of the knee cap (patella) when the quadriceps muscles of the thigh contracts. This movement is best observed by standing on the leg, locking the knee and contracting the thigh muscles. The nerve used is the femoral nerve, and the lack of this twitch is used as a diagnostic sign when applying a femoral nerve block during surgery. This is the left knee. For the right knee, see C016/6803.