GP testing the knee reflex response of a patient. The knee jerk, or patellar reflex, involves stretching the muscle at the front of the thigh by tapping its tendon below the knee cap (patella), and causing the leg to kick. The reflex tests nervous connections between sensory nerves in the muscle, the spinal cord, and motor neurones running from the spinal cord back to the thigh muscle. Absence of a reaction may indicate disease or damage to the area.