The Knee Jerk Reflex, also known as the Patellar Reflex, is a sudden kicking movement in response to a sharp tap to the area below the patella or kneecap. When you strike the patella, it stimulates receptors in the quadriceps muscle in the thigh, sends a signal to the spinal cord, which sends back signals to the muscle fibers, causing them to jerk. An abnormal Knee Jerk Reflex may indicate damage to the central nervous system.