What’s actually cracking when you crack your knuckles?

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. (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.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.