What happens when we crack our knuckles?

For the first time, scientists have proved that the notorious popping sound is the formation of a cavity inside your joint.

To settle what had become a fiery debate among scientists, highlights of which include a vapor bubble formation theory penned in 1947, a team of international researchers pulled fingers under magnetic resonance imaging (MRI) scanners.

They needed someone capable of cracking knuckles on command, and chiropractor Jerome Fryer, who is capable of cracking the knuckles in every finger, again and again after sufficient recuperation time, shared his expertise.

It was Fryer himself who had approached lead author Greg Kawchuck of the University of Alberta, Canada with a new theory on knuckle cracking, and they decided to conduct the study together.

Participant-author Fryer inserted his fingers, one at a time, into a tube that was slowly pulled by a connecting cable until the knuckle gave way to the pop.

During this time the MRI video recorded the cracks, each of which occurred in less than 310 milliseconds.

Each time the joint separated, a gas-filled cavity formed within the lubricating synovial fluid that lets the joints move.

"It's a little bit like forming a vacuum," says Kawchuk. "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."

Why do knuckles crack?

Speculations abound that it's a sign of healthy joints, yet scientists say the amount of force at work during a knuckle cracking has enough power to cause damage to a hard surface.

Still, studies have shown that habitual knuckle cracking does no long-term harm.

Kawchuck and his team are planning more research to explore these conflicting theories and a new mystery they stumbled upon during the study in question.

In what they think represents a first in scientific history, they observed a white flash that appears just before the crack.

Kawchuck says it could be water being drawn together but would need even more sophisticated MRI technology to understand it.

"The ability to crack your knuckles could be related to joint health," he says, remarking that the studies could be important for understanding how to keep all the joints in the body healthy including the spine.

The study was published in the journal PLOS One.