Tendon Reflex

Involuntary responses are ones that you can’t control, but they are usually in place to help with survival. One good example is when you touch something hot. Your hand does not take the time to send a message to your brain and then have the brain tell your hand to pull away. By then, your hand might be seriously hurt! Instead, your body immediately removes your hand in order to protect it from further harm.

Today you will test an involuntary reflex by using the tendon reflex test. A thick, rubbery band called the patellar tendon holds your knee cap in place. Having one leg on top of the other not only stretches the tendon, but it also makes it possible to see a reaction. You can test the reflex by giving your tendon a tap and watching what happens.

Here’s what you need

    1. 1 knee
    2. 1 partner

Here’s what you do

  1. Sit with your legs crossed at the knee on the edge of your seat. Reach forward and see if you can feel the patellar tendon. It is right below your knee cap.
  2. Ask your partner to gently tap the tendon with the outside edge of their hand. This will look like a careful little karate chop. If your partner gets the right spot it will be obvious. You will notice your leg kick out a little in a reflex reaction.
  3. Your partner can try other spots on the tendon if reaction isn’t achieved at first. If it hurts, stop right away! It’s possible that you might not have a tendon response reflex. Not everyone does and that is perfectly normal.

What’s going on?

There are three main parts that make up your peripheral nervous system. They are the autonomic nerves, which control reflexes like the one we have studies here. Autonomic nerves also send information to your organs, blood, and other parts of the body. The second part of your peripheral nervous system is made up of the nerves that deal with the five senses. The last part is your motor nerves. They help you to move the muscles in your body and are responsible for voluntary reactions.

The tendon reflex is in place because the knee is such a sensitive and vulnerable part of the body. When the tendon is stretched out and bumped, your body tries to move the leg and knee out of harm’s way so that it won’t get hurt. As you could probably tell, it’s an involuntary response that neutralizes any conscious, voluntary control that your brain has over the leg through the motor nerves.


  1. What are the main parts of the nervous system?
  2. What are the two parts of the peripheral nervous system and what are their functions?
  3. Which part of the nervous system controls involuntary reflexes?