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Why knuckles crack: A bubble forming in the fluid within the joint when the bones separate.

Why Knuckles Crack : Shots - Health News : NPR

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Yeah science!

The crack apparently comes from a bubble forming in the fluid within the joint when the bones separate, according to a study published Wednesday. It's like a tiny air bag inflating.

The findings confirm the original theory about knuckle-cracking, which was first proposed in 1947 but challenged in the 1970s.

According to Greg Kawchuk, a professor of rehabilitation medicine at the University of Alberta, that second group of scientists came along and said, " 'No, no, no — wait! We think what's happening is, the gas bubble forms but then it subsequently collapses. That's what makes the big sound.' "

While many people accepted the bubble-bursting theory, no one was sure. So Kawchuk and a team of scientists figured out a little test, enlisting the help of a pal who is really good at cracking his knuckles.

"We called our colleague the 'Wayne Gretzky of finger-cracking,' " Kawchuck says. "He can make this happen in all 10 of his fingers."

They asked the volunteer to put his hand inside a special type of MRI scanner, and made a movie of the inside of his knuckles as they pulled on the end of each finger to make it crack.

"We've been calling it the 'Pull My Finger Study,' " Kawchuk says.

What they saw was clear: The cracking sound comes when a bubble forms between the bones of the knuckle joint — not when it collapses.

"Our jaws hit floor," he says. "This is the exact answer. It feels pretty great."

Other researchers praised the study, which appears in the online journal PLOS ONE.

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