Knuckle cracking: scientists explain what's really going on

An international team of scientists claims to have solved the decades-long debate of what happens when people crack their knuckles.

Using MRI video, scientists found that the "popping" sound was caused by a gas-filled cavity forming rapidly within the fluid surrounding the joint.

A similar conclusion was drawn in a 1947 study, but research in 1971 claimed the noise was related to the bubble collapsing rather than forming.

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

Kawchuk likened the process to forming a vacuum. "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," he explained.

Canadian chiropractor Jerome Fryer, who can crack all of his fingers on demand, was used as a test subject, with his fingers inserted one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked.

The MRI video, which showed each 310-millisecond knuckle crack in real time, revealed a white flash just before the cracking. Kawchuk said the white flash, which had never been observed before, could be water suddenly drawn together.

The team hopes that the findings, published in scientific journal Plos One, will pave the way for new research into the benefits or dangers of knuckle cracking.

Previous studies provided conflicting results, with some saying habitual knuckle cracking does not appear to cause long-term harm and others showing that the force of knuckle cracking has enough energy to damage hard surfaces.

Kawchuk said future research on knuckle could also offer insight into other joints in the body, including the spine, and help explain why they become arthritic or injured.