The nervous system code for the difference in intensity between a gentle tap with a hammer and a hard blow with the hammer, is that the rate of neural firing and the number of neurons firing. Reflex hammer is a medical instrument used by practitioners to test deep tendon reflexes. Reflex tests are performed as part of a neurological exam, either a mini-exam done to quickly confirm integrity of the spinal cord or a more complete exam performed to diagnose the presence and location of spinal cord injury or neuromuscular disease.