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. It's used as a test for nerve damage. The test has become so well-known that it entered the American lexicon as a saying.

Triggering the knee jerk reflex involves using a soft rubber reflex hammer. Simply striking 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.

The knee jerk 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 wills it. This is because the nerves in it connect directly to the spinal cord rather than the brain. The knee jerk reflex test is thus used to check for nerve damage in the nerve system as a whole.

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

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 knee jerk reflex time can also be altered by external conditions. For example, some studies have reported that the reflex 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 their unconscious mind.

The knee jerk reflex text has become embedded in popular culture as an idiom. 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. Since its first appearance, the phrase “knee jerk response” has come to mean reacting to something with prejudice or making assumptions about something or someone with as little control as the brain has over the patellar tendon.