MODEL RELEASED. Reflex test. General practice (GP) doctor testing a boy's knee reflex. He is using a hammer-like device to gently hit a specific area just below the boy's knee. This should trigger an involuntary muscle contraction that will move the boy's lower leg. Such an involuntary response is called a nervous reflex. It is used to test the health of the muscular and nervous system. If the reflex is absent, this may indicate damage to the nerves, muscles or tendons.