What is a Patellar Reflex?

Mary McMahon
Mary McMahon

The patellar reflex is a type of deep tendon reflex that occurs when an area just below the patella, also known as the kneecap, is struck. In healthy individuals, when the right spot is tapped, this causes the lower leg to kick out almost instantaneously. Medical professionals may check this reflex during a routine neurological exam, looking for responses that are exaggerated, delayed, or not present.

Doctor checking a child's knee reflexes.
Doctor checking a child's knee reflexes.

This reflex is what is known as a monosynaptic reflex, because only one synapse needs to be crossed to complete the circuit that triggers it. When the area below the kneecap is hit with a reflex hammer, it hits the patellar tendon, which causes the quadriceps muscle in the thigh to contract, leading the leg to kick out. This involuntary response does not involve the brain, only the spinal cord, and while it feels instantaneous to the observer, around 50 milliseconds are actually involved in the response time, as people would see if they saw a radically slowed film of the event.

Doctors use a reflex hammer to test the patellar reflex.
Doctors use a reflex hammer to test the patellar reflex.

If someone does not have a patellar reflex, he or she is said to be exhibiting Westphal's sign. This indicates that there is a problem in the patient's spinal cord or peripheral nerves. The healthcare professional usually assesses the reflex on both legs to see the extent of the problem. It is also possible for a patient to experience an exaggerated reflex, in which the leg kicks out more radically than would be expected.

The patellar reflex is what is known as a monosynaptic reflex, because only one synapse needs to be crossed to complete the circuit that triggers it.
The patellar reflex is what is known as a monosynaptic reflex, because only one synapse needs to be crossed to complete the circuit that triggers it.

A number of reflexes can be used to assess physical and neurological health. Patellar reflexes provide information about specific nerves in the leg involved, along with the spinal cord, and they may be used in routine physicals to check on a patient's health, as well as in specific neurological exams to explore possible causes for neurological symptoms. If the reflex is abnormal, a medical professional may recommend additional testing to learn more about the cause of the abnormality, and to start developing a diagnosis, along with treatment options.

"Patella" is the medical term for what is commonly known as the kneecap.
"Patella" is the medical term for what is commonly known as the kneecap.

This particular reflex is so well-known that the common name for it, “kneejerk reflex,” is sometimes used to describe a situation in which someone responds to something without really thinking. In a metaphorical kneejerk response, someone can lash out verbally instead of kicking physically, sometimes causing social tension. This reflex can also be observed in animals, and it is used in routine neurological screening by veterinarians as well.

A patellar reflex may be conducted during a routine neurological exam.
A patellar reflex may be conducted during a routine neurological exam.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments

anon359103

Can a patellar reflex go away and then return? Or, once the reflex disappears, is it permanent?

anon287851

I'm doing this this test for my biology project. How do you test this based on age?

BambooForest

When children are little they often can be bored or even scared by the need for the reflex tendon test on the knee every time they go to the doctor, but when you get older and realize how dangerous a problem you might have if your knee has no patellar reflex, suddenly that one hit with the little hammer does not seem like so big a deal.

Catapult

Another name for the knee jerk reflex is the "knee jerk reaction", which like the article says is sort an idiom for a first impression or first reaction to something shocking or otherwise surprising.

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