What is it that makes that popping sound when you crack your knuckles? A team of researchers led by the University of Alberta Faculty of Rehabilitation Medicine have confirmed that it is because of vacuum cavities forming in the joint's synovial fluid.

How? By pulling the fingers of a test subject inside an MRI machine.

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

The idea for the study came from Nanaimo chiropractor Jerome Fryer, who approached Professor Kawchuk with a theory. Rather than beat around the bush, they decided to take a natural look using magnetic resonance imaging.

"Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team," Professor Kawchuk said.

Fryer's fingers were inserted, one at a time, into a tube attached to a cable; this tube slowly pulled on each finger until the knuckle cracked. And in each instance, it was the bubble formation in the synovial fluid was associated with the popping sound, occurring within 310 milliseconds.

"It's a little bit like forming a vacuum," Professor Kawchuk explained. "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 data fail to support evidence that knuckle cracking leads to degenerative changes in the metacarpal phalangeal joints in old age," the study concludes.