MODEL RELEASED. Knee reflex test. Physiotherapist using a testing mallet to perform a knee reflex test on a young child. The mallet is used to gently tap a specific area just below the child's knee. This should trigger an involuntary muscle contraction that will move the child'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.