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.
Illustration by Slug Signorino
Cecil would love to put you out of your misery, Clive. (Thirty cracks? Gross.) But you’re pretty much on the money about gas bubbles popping. One beef: they don’t explode, they implode, a matter of 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 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. Stop before it’s too late.
Send questions to Cecil via firstname.lastname@example.org.