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

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?

But of course this is also a joke. You don’t want to overdo the kidding with a profession that’s legally empowered to remove your aorta. The actual purpose of knee tapping is to test for pathological conditions that, while not common, are far from nonexistent. These conditions fall into two categories. (1) Hyperactive deep tendon reflex (knee jerks too much): amyotrophic lateral sclerosis, brain tumor, cerebrovascular accident (stroke), hepatic encephalopathy (associated with liver disease), hypocalcemia (low calcium), hypomagnesemia (low magnesium), hypothermia, multiple sclerosis, preeclampsia, spinal cord lesion (e.g., tumor), and tetanus. (2) Hypoactive DTR (knee doesn’t jerk enough): botulism, Eaton-Lambert syndrome, Guillain-Barre syndrome (nerve inflammation), peripheral neuropathy, polymyositis, syringomyelia, tabes dorsalis, and other ailments too scary to pronounce much less have.

When the doctor tests your reflexes she’s tapping the tendon that connects the muscle to the bone, which causes the muscle to stretch slightly. This sends a nerve impulse to your spinal cord, where it triggers a motor impulse that returns via a parallel nerve and causes the muscle to twitch.

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.


The Straight Dope of May 19 discussed why Great Britain is called “great.” I always thought the island was called Great Britain to distinguish it from Brittany (“Little Britain”) on the mainland. In French these places are called Bretagne and Grand Bretagne, and the Celtic people from both places are called Bretons. –Michael Arscott, Montreal, Quebec

The Teeming Millions have been contradicting their Uncle Cecil a lot lately, haven’t they? Shows you the continuing moral breakdown of our society. Nonetheless your point is well taken. I wrote that the term Britain, used in Roman times, was resurrected in the 16th century as the name for the projected union of England, Scotland, and Wales. Fascinating, but when you get right down to it, totally irrelevant. The reason they called it Great Britain and not just Britain was to distinguish it from Brittany, aka Britannia minor, lesser Britain, the French peninsula that had been settled in the fifth and sixth centuries by Celtic immigrants from the British Isles.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.