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What is a Knee Jerk Reflex?

By Erica Stratton
Updated Mar 03, 2024
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The knee jerk reflex is when a tendon above the kneecap is tapped with a reflex hammer, which causes the whole leg to involuntarily jerk. Triggering the reflex is relatively simple, although just hitting above the kneecap won't cause the reflex — the patellar tendon must be tapped. This causes the tendon's nerve receptors to fire, and the leg will jerk involuntarily. This test for nerve damage has become so well-known that it entered the American lexicon.

The reflex is famous because of the way it illustrates the different types of nerve pathways in the body. The patellar tendon will move when pressure is put on it whether or not the patient wants it to. This is because the nerves there connect directly to the spinal cord rather than the brain. The knee jerk reflex test is therefore used to check for nerve damage in the nerve system as a whole.

The different kinds of responses a medical professional will get when striking the patellar tendon can be graded. Legs that do not jerk at all or that jerk continuously after being struck are considered abnormal. A healthcare professional will compare the different responses he will get in each leg to determine whether there is damage in only one.

Complete lack of a knee jerk reflex can be a sign of several nerve conditions and is referred to as "Westphal's sign," after the 18th century physician Carl Friedrich Otto Westphal. It can be a sign of spinal damage, injury to the femoral nerve, or multiple sclerosis. It may even be a result of a thyroid disease, which can alter all motor reaction times.

The reflex time can also be altered by external conditions. For example, some studies have reported that the time is shorter after the patient has exercised. Another unusual link is that a distracted person may have more of a patellar reflex, as if the unconscious function was reflecting his or her unconscious mind.

This test has become embedded in popular culture as an idiom with the phrase "knee jerk reaction." It is used to mean reacting to something with prejudice or making assumptions about something or someone with little thought. Its first use was in the 1800s, when a preacher’s words were compared to a hammer striking the knees of believers, but not getting any response.

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Discussion Comments

By anon327449 — On Mar 27, 2013

It would be helpful if the article reported what an exaggerated knee jerk response indicates.

By anon303576 — On Nov 15, 2012

@EarlyForest: The updated name for the Achilles tendon is now the Calcaneal Tendon, as this is actually called the Calcaneous Tendon. There is a reflex for it, but my information is too limited to answer any further.

Also, the signals for the "knee jerk" or Patellar Reflex do not depend on interneurons. Just do some research on the Patellar reflex. The knee jerk reflex is an example of a monosynaptic or "stretch" reflex.

By anon294891 — On Oct 03, 2012

I'm just wondering. I had knee jerk in my right leg but nothing at all in my left. What could cause this, or is there a reason for this?

By galen84basc — On Jan 01, 2011

Hello all. I was wondering what it could mean if I had no knee jerk reflex at times. It's really weird, because sometimes I have a very obvious knee jerk reflex and it's very easy to get it to happen, but other times I really can't seem to get it at all.

Is there some sort of condition that could cause a person to only have a knee jerk reflex at certain times and not other times? For instance, is there some condition that can affect a tendon's reflex response, or even the knee jerk reflex pathway?

I would be very interested to learn more about this, so I'd appreciate any information. Thank you in advance for your information.

By pharmchick78 — On Dec 30, 2010

Good article. So many patients come into my clinic and think that the knee jerk reflex is some kind of magic. While that's cute in a 5 year old, it's less so in a 30 year old.

By the way, did you know that there are other ways to check spinal reflexes too? It comes down to testing certain reflex tendons.

For example, aside from testing the knee jerk reflex, doctors can also check the ankle jerk reflex by checking your achilles tendon reflex. Doctors can also check your plantar reflex by tapping your foot.

It's always interesting to see how different patients react to that too -- everybody is used to seeing their knee reflex, but it can be surprising to see your ankle start to jump like that!

By EarlyForest — On Dec 28, 2010

Hi -- thank you for your article. I have a bio worksheet on the nervous system that is just killing me, and I was wondering if you could help me answer some questions on it...I know, I know, I should do my own homework, but a guy can ask for help, right?

First question: The knee jerk reflex is controlled by interneurons in the...?

And secondly: The knee jerk reflex is an example of a...?

Oh, also, is there such a thing as an achilles tendon reflex? That's a true/false question.

Thank you for your help, and for your ever-informative articles.

Best, EF

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