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health advisory Hospital Doctor Yu 2007-11-8 19:08:29

Answers:

It has been incorrectly attributed to synovial fluid moving suddenly (popping) from one side of the synovial fluid cavity to the other when the one side is under pressure.

It is ACTUALLY the sudden stretching and 'relaxing' of your tendons over your joints as you put the tendons under tension when snapping your fingers.

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Friction

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It's the fluid in between the bones that makes the sound. I worked for a chiropractor.

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Uncle Cecil speaks:

"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."