Why is it when a doctor gives you a physical examination, he taps your knee with a rubber hammer? My knee always jerks when he does this, and the same goes for everyone I have ever spoken to. Which makes me wonder if anyone has ever failed it, and what became of them. Does the medical community just go on looking, looking, hoping to find a person who fails the test? Or is there actually some hideous disease which has as one of its early symptoms that your knees do not jerk when struck? –Bill Kinnersley, via the Internet
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I consulted the Straight Dope Science Advisory Board about this, and one member informed me that if no knee-jerk reflex can be elicited “this is one of the diagnostic signs that the patient is dead.” This is, I believe, a joke. My personal opinion is that knee tapping comes under the heading of “card tricks to impress the customers.” Look! I tapped your knee! It jerked all by itself! Am I a medical genius or what?
A faulty reflex in itself is not conclusive evidence that you have one of the problems above. For example, in the case of preeclampsia, a form of hypertension, you also have to be pregnant. But a bad reflex does tell the doctor to investigate further. One way to do this is by testing other reflexes. The doctor usually starts with your knee-jerk response, also known as the patellar reflex, because it’s quick and easy. But she can also whale away on your elbow (triceps reflex), crook of your arm (biceps reflex), wrist (brachioradialis reflex), or back of your ankle (Achilles tendon reflex). If you’ve got feeble reflexes all over plus muscle weakness and blurred and double vision, maybe you’ve got botulism. If you’ve got hyperactive reflexes on only one side of the body, that’s a sign of brain tumor or stroke. If your patellar reflexes eat but your triceps reflexes are OK, that may mean you’ve got a lesion (injury) between your second lumbar vertebra and your–ah, hell, only your doctor needs to know the details. Just be thankful, next time you cast a jaundiced eye on that little hammer, that she knows.
