The Real Reason Your Knuckles Crack Finally Revealed

— -- A team of researchers set out to discover what makes knuckles crack, and at long last, it seems they found the answer.

(Spoiler alert: It has nothing to do with air bubbles "popping.")

A team of researchers used real-time MRI imaging to look "inside" fingers as they cracked and popped, according to a study published this week in the medical journal PLOS ONE. Since they needed someone who could crack their knuckles on command, they decided to use study co-author Jerome Fryer.

"Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team," lead author Gregory Kawchuk, a physical therapy professor at the University of Alberta in Canada, said in a statement.

Kawchuk's team connected Fryer's fingers to a device that pulled on them to crack them, and they watched to see what happened as the joints separated, according to the study. The researchers found that when the force pulling the bones apart overwhelms bones' ability to stick together, a cavity forms in the liquid that lubricates the joint and the pressure drops. A gas fills the cavity but it's not air from outside the joint -- it comes from dissolved gas coming out of the joint lubricant.

Although the experiment explains the sound of knuckles cracking, it doesn't explain the magnitude of that sound, the study authors wrote.

Dr. Allison Yang from the ABC News Medical Unit contributed to this report.