The Real Reason Doctors Hit Your Knee

During your annual physical exam, you've probably experienced that weird reflex when the doctor taps your knee and your foot automatically kicks without you having to give it a thought. This test, called a patellar reflex test, activates the tendon just below the kneecap called the patellar tendon (via University of Michigan Health). You may know that the test has something to do with reflexes, but what exactly is your doctor looking for when hitting your knee?

The answer has to do with your nervous system. When your physician taps the tendon, your nerves respond before your brain has a chance to think. That is what makes this test important. Once the nerve feels the tap, it sends an instantaneous message to the spinal cord, which in turn sends an immediate message back to the nerve, which causes you to kick (via MedicineNet). The physical response represents how your nerves and your spinal cord communicate. More specifically, the tap tests the L2, L3, and L4 regions of the spinal cord.

When the leg does not respond with a kick or with a smaller than normal movement, it is referred to as "Westphal's Sign," named after the doctor who observed it in the late 1800s (via PubMed). While the absence of a reflex to the patellar reflex test is considered abnormal, that does not necessarily mean something is wrong. In fact, many healthy people get an abnormal result (via The National Center for Biotechnology Information).

However, if you have no reaction to the patellar reflex test, it might indicate a neurological disorder involving your peripheral nervous system. If your knee kicks more than once, you could have what is called a hyper-reflexive response to the test, which might indicate problems with the central nervous system. If you do have an abnormal result on a test, your doctor will generally run a series of other tests to help determine if there is a problem that needs attention (via Verywell Health).