MODEL RELEASED. Knee jerk reflex test. General practice doctor using a patellar hammer to test a patient's knee jerk reflex. Tapping just below the knee cap (patella) stretchs the thigh muscle. The resulting nerve impulse passes along a sensory neurone (nerve cell) to the spinal cord, where a motor neurone is activated. This causes the muscle to contract and the leg to kick up. This process takes about 50 milliseconds to complete. The reflex tests the nervous connections in this reflex arc. An absence of a knee jerk reflex may indicate damage to the nervous system.