When you crack your knuckles, you are creating a gas bubble between the bones of the pulled or flexed joints, suggests new research that helps to solve a long-standing mystery over what produces the familiar "crack" sound.
The sound is not all that results, because the research - presented today at the annual meeting of the Radiological Society of North America in Chicago - also confirmed prior speculation that a flash of light happens about 10 milliseconds after the sound.
"What we saw was a bright flash on ultrasound, like a firework exploding in the joint," radiologist Robert Boutin of University of California at Davis said in a press release. "It was quite an unexpected finding."
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Boutin, who led the research, and his colleagues used ultrasound imaging to simultaneously record the audio and visuals of knuckles cracking. The participants consisted of 17 women and 23 men ranging in age from 18 to 63. Thirty said that they regularly cracked their knuckles, while the other 10 said that they did not.
Those who did so regularly reveal what a habit this can be: they said they have cracked their knuckles for up to 20 times a day for years on end. The fact that these people seem to have healthy joints appears to provide evidence that knuckle cracking for people without other joint problems does not cause much, if any, damage.
"We found that there was no immediate disability in the knuckle crackers in our study, although further research will need to be done to assess any long-term hazard - or benefit - of knuckle cracking," Boutin said.
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In terms of what exactly occurs as knuckles crack, he said, "There have been several theories over the years and a fair amount of controversy about what's happening in the joint when it cracks."
"We're confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."
Since the bubbles remain there for 20–30 minutes after the crack, the theory that the noise results from a gas bubble bursting has been busted. Instead, when a person separates bones that make up a joint, it looks like a void results where the temporary gas bubble subsequently forms.
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Knuckle cracking happens unintentionally from time to time with regular use of our hands, so it is possible that the phenomenon helps safeguard the joints in some way.