MODEL RELEASED. Knee jerk reflex. Paediatric doctor tests the knee jerk reflex response of a young boy. A patellar hammer is used lightly for this purpose. The knee jerk, or patellar reflex, involves tapping the tendon below the knee cap (patella); this stretches the tendon associated with one of the thigh muscles, causing the muscle to contract and the leg to kick up. 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 knee jerk reflex reaction may indicate disease or damage to the local area, or to the central nervous system.