Tendon reflex, like the patella reflex so often checked by doctors, tapping with the little rubber hammer, is an involuntary response to stimulus that prompts automatic reaction of a limb or body part. Some reflex actions like the patellar reflex operate in a closed neural circuit, called a "reflex arc", that saves time and thought, bypassing the conscious and immediately eliciting a quick response.
The relative health status of some individuals is also reflected in involuntary responses. When the doctor checks my patellar reflexes, watch out!, because some of them are hyperactive. Especially the right leg, which exhibits what is called "tonic" reflex, or just "tone" for short. Too much "tone" means that flexors and extensors are opposing each other too much to allow freedom of voluntary movement. When the doctor taps lightly on the ligament below the patella, my leg responds in a fairly dramatic way.
