Scientists determined exactly what happens when a person cracks their knuckles for the first time.

A recent study found the distinctive knuckle-popping sound is caused by a cavity that rapidly forms within the joint, the University of Alberta reported.

"We call it the 'pull my finger study'--and actually pulled on someone's finger and filmed what happens in the MRI. When you do that, you can actually see very clearly what is happening inside the joints," said lead author Greg Kawchuk, a professor in the Faculty of Rehabilitation Medicine.

Scientists have been debating what causes knuckles to crack since 1947, when a team of U.K. researchers suggested it was caused by vapor bubbles forming in the joints; in the 1970s another team of scientists said it was more likely to be caused by collapsing bubbles.

To make the findings Nanaimo chiropractor Jerome Fryer (who can crack every finger on demand) inserted his fingers into a tube connected to a cable that was pulled until the digit cracked. An MRI video revealed what was occurring within the hand during the experiment in real time. Each time, the cracking was correlated with a gas-filled cavity within the synovial fluid, which lubricates the joints.

"It's a little bit like forming a vacuum," Kawchuk said. "As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what's associated with the sound."

The team also noticed the presence of a "white flash" that occurs just before cracking, which could suggest water is being drawn together in the joint. In the future the researchers plan to investigate how these new findings influence ideas regarding knuckle-cracking's effect on health. 

"It may be that we can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin," Kawchuk concluded.
The findings were published in a recent edition of the journal  PLOS ONE.