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Scientists Explain the Sound of Knuckle Cracking (bbc.com)
"The BBC reports on something sure to impress your next date -- and possibly your last -- when you explain it," writes Slashdot reader dryriver. From the report: Scientists have turned their attention to investigating that most annoying of human habits -- the sound made when you crack your knuckles. The characteristic pop can be explained by three mathematical equations, say researchers in the US and France. Their model confirms the idea that the cracking sound is due to tiny bubbles collapsing in the fluid of the joint as the pressure changes. Surprisingly, perhaps, the phenomenon has been debated for around a century. Science student Vineeth Chandran Suja was cracking his knuckles in class in France when he decided to investigate.
"The first equation describes the pressure variations inside our joint when we crack our knuckles," he told BBC News. "The second equation is a well-known equation which describes the size variations of bubbles in response to pressure variations. And the third equation that we wrote down was coupling the size variation of the bubbles to ones that produce sounds." The equations make up a complete mathematical model that describes the sound of knuckle cracking, said Chandran Suja, who is now a postgraduate student at Stanford University in California. "When we crack our knuckles we're actually pulling apart our joints," he explained. "And when we do that the pressure goes down. Bubbles appear in the fluid, which is lubricating the joint -- the synovial fluid. "During the process of knuckle cracking there are pressure variations in the joint which causes the size of the bubbles to fluctuate extremely fast, and this leads to sound, which we associate with knuckle cracking.'' The study has been published in the journal Scientific Reports.
"The first equation describes the pressure variations inside our joint when we crack our knuckles," he told BBC News. "The second equation is a well-known equation which describes the size variations of bubbles in response to pressure variations. And the third equation that we wrote down was coupling the size variation of the bubbles to ones that produce sounds." The equations make up a complete mathematical model that describes the sound of knuckle cracking, said Chandran Suja, who is now a postgraduate student at Stanford University in California. "When we crack our knuckles we're actually pulling apart our joints," he explained. "And when we do that the pressure goes down. Bubbles appear in the fluid, which is lubricating the joint -- the synovial fluid. "During the process of knuckle cracking there are pressure variations in the joint which causes the size of the bubbles to fluctuate extremely fast, and this leads to sound, which we associate with knuckle cracking.'' The study has been published in the journal Scientific Reports.
Miss-attribution. The pressure variations both cause the bubble and cause them to collapse suddenly. It's called cavitation. We don't normally have bubbles in our joints and we don't build up any over time either.
What does build up in the sinovial fluid is the nitrogen gas which is pulled out of solution (forming bubbles) and then released. After the gas is released, you have to wait a while for gas to migrate from surrounding tissues (by osmosis) before you can crack again.
I've noticed that after loading up a significant amount of nitrogen from repeated deep SCUBA dives, my joints become considerably more "crackly". It takes less motion to pop them, and they can be popped again in a shorter amount of time than normal. Some people tell me they get the same effect just by taking a plane ride that starts at sea level, so the 8000-foot cabin pressure creates enough ppN2 differential to increase cracking. Doesn't do that for me, though.
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When someone says, "Any fool can see
The entire thread replying to the first comment posted here seems to be ignoring the content of that comment: this is not new. Some of the detailed analysis might or might not be new, but the main result is known.
Here are some popular articles from years back:
https://www.telegraph.co.uk/news/science/science-news/11539913/Why-knuckle-cracking-makes-a-popping-sound-and-why-it-might-be-beneficial.html : Quote: "When muscle joints are pulled apart there forms a tiny cavity filled with gas which then collapses, creating a popping noise."
https://health.howstuffworks.com/human-body/systems/musculoskeletal/question437.htm
https://articles.mercola.com/sites/articles/archive/2014/11/08/cracking-knuckles-harmful.aspx