Knee jerk: The reflex tested by tapping just below the knee causing the lower leg to suddenly jerk forward.

What is tapped to elicit this reaction is the patellar tendon , the tendon that runs down from the quadriceps muscle in the front of the thigh, over the kneecap (the patella ), down to the lower leg. And what happens is that the quadriceps contracts and abruptly brings the lower leg forward. This reaction is involuntary since it occurs without the person willing it to happen.

The knee jerk is best tested by giving a smart tap on the patellar tendon while the lower leg hangs loosely at a right angle with the thigh . The test is part of the clinical neurologic examination. The knee jerk is a deep tendon reflex (DTR). (The ankle (Achilles tendon) jerk is another.) DTRs can be graded:

• zero absent
• 1+ hypoactive (underactive)
• 2+ "normal"
• 3+ hyperactive (overactive) without clonus (extra jerks)
• 4+ hyperactive with unsustained clonus (just 1 or 2 extra jerks)
• 5+ hyperactive with sustained clonus (continued jerking)

An asymmetric reflex in which one knee jerk on one side is different than that on the other side is abnormal. So is the spread of tendon reflexes (tapping one knee and observing contraction of muscles elsewhere). And sustained clonus is also distinctly abnormal. Absence of the knee jerk can be due to an abnormality in the "reflex arc" required for the reflex to occur (the muscle "spindles" or the nerve fibers going from the patellar tendon to the spinal cord and returning from the spinal cord to the quadriceps). With a stroke, the knee-jerk reflex may at first be underactive, then recover and become hyperactive within a day or two.

The knee jerk has been so often tested and become so familiar that it has given rise to the adjective "knee-jerk" as in a knee-jerk reaction. Knee-jerk in this figurative sense means "readily predictable to the point of being automatic." It often has a negative connotation and conveys the idea of an all-too-hasty, impulsive, irrational response based on a preset idea. For example, a dictator's knee-jerk response to a democratic movement is to suppress it.

The knee-jerk reflex is also medically called the patellar reflex. It is less often referred to as the knee phenomenon, the knee reflex, the patellar tendon reflex, or the quadriceps reflex.

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