MODEL RELEASED. Knee jerk reflex. General practice doctor using a patellar hammer to test a woman's knee jerk reflex. Tapping just below the knee cap (patella) will stretch a tendon of one of her thigh muscles. 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. The reflex tests the nervous connections in this reflex arc. An absence of a knee jerk reflex may indicate damage to the nervous system.