Scientists equipped with an MRI scanner, a finger-pulling device, and a man they call the "Wayne Gretzky of knuckle-cracking" say they've cracked the mystery of that popping sound your knuckles make when cracked. In a University of Alberta press release, the team says their video reveals that the sound is caused by the formation of a gas-filled bubble created by a drop in pressure when the joints are separated. "We call it the 'pull my finger study'—and actually pulled on someone’s finger and filmed what happens in the MRI," the lead researcher says. "When you do that, you can actually see very clearly what is happening inside the joint." The study was published in the journal PLoS One.
The bubble comes from the fluid that lubricates the joints, the lead researcher says. "If you've ever washed up glass plates, you'll know they can be hard to separate when they are wet," he tells the Guardian. "The film of water between them creates a tension that needs to be overcome. It's similar with joints. When you pull on them, they resist at first, and then suddenly give way." He says he started the study after a local chiropractor—who served as their champion knuckle-cracker—approached him with the theory. The researcher, an expert in spinal structure, says the findings could help explain the cause of joint problems and help doctors stop them before they begin. (Other researchers have discovered a new body part in the knee.)