If you've ever been to the doctor for a check-up, you may have experienced a simple test involving a hammer. No, the doctor doesn't use a hammer like you might find in your toolbox in the garage. Instead, the doctor's hammer is small and has a rubber tip.
The doctor probably used his little rubber hammer to tap your knee. When he did that, your leg probably kicked forward before you even realized what was happening. What's going on here? The doctor was simply testing your reflexes.
Reflexes are involuntary actions your body takes in response to certain stimuli. That means they're automatic. Reflexes occur without your having to think about it. Reflexes are simply natural reactions we're all born with. In fact, they're a sign you're healthy. That's why doctors test for them.
For example, have you ever picked up something that was hot enough to burn your hand? Before your brain can even register the fact that what you picked up was too hot, your body has already acted via a reflex to remove your hand from the source of heat.
You're definitely familiar with a couple of other reflexes…even if you didn't realize they were reflexes. Since you've been reading this Wonder of the Day, you've probably blinked several times. Blinking is a reflex that protects your eyes by keeping dust and other particles out while also keeping your eyes moist.
If you've ever sneezed or coughed, you've experienced other reflexes. When irritating particles get into your breathing passageways, sneezing and coughing are both reflexes that help to protect your air passageways by keeping unwanted particles out.
When a doctor taps your knee with his rubber hammer to make your leg kick, he's actually testing what's known as the patellar reflex. It's also called the deep tendon reflex (DTR), because the doctor actually is tapping on your patellar tendon.
The tap on the tendon stretches the tendon, which in turn stretches the thigh muscle connected to it. This message is sent to the spinal cord, and the spinal cord immediately sends back a message that tells the muscle to contract. This is what causes your leg to kick!
Other reflexes might have a more obvious protective function than the patellar reflex, but it's very important nonetheless. Without the patellar reflex, you might have trouble standing and keeping your balance. From time to time, the forces of gravity can cause your knees to bend slightly. The patellar reflex keeps you standing tall instead of falling down.