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A Straight Dope Classic from Cecil's storehouse of human knowledge
09-Nov-1984
Dear Cecil:
Every time I crack my knuckles the kids in my classes wince and gleefully inform me that I
will get arthritis. My yoga book, on the other hand, informs me that the practice is
beneficial. Who's right? I once read that the cracking sound was due to gas bubbles being
exploded inside the joints. Is this true? When I was a grotty child I used to crack not
only my finger joints but my toe joints also. I used to consistently reach 30 cracks, but
have always wondered whether there was a maximum possible number of cracks or whether
there is a standing world record. Uncle Cecil, put me out my misery and give us the
straight dope. --Clive G., Baltimore
Cecil replies:
Cecil would love to put you out of your misery, Clive, since he regards knuckle-cracking
as a crime against nature, but once again we find justice is obstructed by the law. Be
that as it may, you're pretty much on the money with this business about gas bubbles
popping. Actually they don't explode, they implode, a matter of some importance to us
scientists.
Here's the deal. The knuckle (MDs call it the metacarpo-phalangeal joint) is surrounded by
the synovial fluid, a clear liquid that lubricates the joint. This fluid contains about 15
percent carbon dioxide in solution. When you crack your knuckles, you tug or twist the
finger or toe with a steady effort, creating a low-pressure zone within the synovial
fluid. According to the most likely hypothesis (Unsworth, Dowson, and Wright, 1971), the
low pressure draws CO2 and water vapor out of solution, creating a bubble. (This process
is called cavitation.) This collapses almost instantly, and the fluid crashing in from all
sides makes the noise.
Once the big gas bubble has popped, a little one remains behind for about 15 or 20 minutes
before the CO2 inside it is totally redissolved. During that time, any further
finger-tugging simply causes the micro-bubble to expand a bit, like a tiny shock absorber.
That's why you can't crack the same knuckle twice in rapid succession. (The fact that the
knucklebones remain at maximum extension for a while is also a factor.) This leads me to
conclude that the theoretical ceiling for knuckle-cracking is ... let me get my shoes off
here ... 56. Sounds like you've still got a ways to go with your 30. Incidentally, not all
cracking noises produced by stretching--e.g., in the backbone--are the result of gas
bubbles popping. Sometimes the noise is caused by a ligament snapping over some bony
projection.
Will cracking your knuckles cause knobbiness and arthritis, as some claim? Not
necessarily; genetics undoubtedly plays a much more important role. But some clinicians
believe chronic knuckle-cracking can make things worse. And it certainly doesn't do much
for your standing in civilized society. Knock it off before it's too late.
--CECIL ADAMS
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