In a study published in PLOS ONE, an international team of researchers led by the University of Alberta used MRI video to determine what happens inside finger joints to cause the distinctive popping sounds heard when cracking knuckles. For the first time, they observed that the cause is a cavity forming rapidly inside the joint.

In every instance, the cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid, a super-slippery substance that lubricates the joints.

“It’s a little bit like forming a vacuum,” said professor and lead author Greg Kawchuk, Faculty of Rehabilitation Medicine. “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.”