Have you ever wondered why your leg kicks when the doctor taps your knee with that little rubber hammer? It's because of your reflex.

A reflex is an involuntary (say: in-vah-lun-ter-ee), or automatic, action that your body does in response to something — without you even having to think about it. You don't decide to kick your leg, it just kicks. There are many types of reflexes and every healthy person has them. In fact, we're born with most of them.

Reflexes protect your body from things that can harm it. For example, if you put your hand on a hot stove, a reflex causes you to immediately remove your hand before a "Hey, this is hot!" message even gets to your brain.

Blinking and Sneezing

Other examples of protective reflexes are blinking when something flies toward your eyes or raising your arm if a ball is thrown your way. Even coughing and sneezing are reflexes. They clear the airways of irritating things.

The reflex that the doctor checks by tapping your knee is called the patellar, or knee-jerk, reflex. It is also known as a deep tendon reflex (DTR) because the doctor is actually tapping on a tendon called the patellar (say: puh-teh-lur) tendon. This tap stretches the tendon and the muscle in the thigh that connects to it. A message then gets sent to the spinal cord that the muscle has been stretched.

The spinal cord very quickly sends a message back to the muscle telling it to contract. The contraction of the muscle causes your lower leg to kick out. You might be wondering why such a reflex exists. This type of reflex is important in keeping your balance. When you're standing up, gravity might cause your knee to bend slightly, and this could make you fall if you didn't have the protective DTR to straighten that knee and keep you standing upright.

A doctor often checks for DTRs to make sure that the nervous system is working properly. Aside from the knee, they also can be checked along the outside of the elbows, in the crooks of the arms, and at the wrists and ankles.

So the next time your doctor taps you with that little rubber hammer, say, "Hey doc, how are my DTRs?"

Reviewed by: Steven Dowshen, MD
Date reviewed: September 2010