The sweet release of cracking knuckles has always baffled scientists. Over the years, scientists trying to explain the cracking sound have pointed to “bubbles” created by rapid pressure changes in the knuckle joints, but they have been unable to determine whether the sound has to do with bubbles popping or bubbles forming. A study published in Scientific Reports on Thursday, however, offers a new mathematical explanation that seems to settle the debate.
The new paper suggests that the loud crack people heard is not, as researchers argued in 1971, the sound of bubbles bursting. Rather, say the researchers, it’s the sound of synovial fluid-filled bubbles rapidly forming.
“When you suddenly change the pressure in that fluid as a result of increasing the spacing between the knuckles, some of the gases in that fluid can nucleate into a bubble,” Abdul Barakat, Ph.D., professor of hydrodynamics at France’s École Polytechnique and co-author of the study, told the Guardian on Thursday. Working with Vineeth Chandran Suja, Ph.D. of Stanford University’s Department of Chemical Engineering, Barakat knew that, to end the debate, they had to figure out how to determine what was actually happening in the joint.
To do so, he first looked at the imaging techniques available to scientists who linked the bubbles to the mysterious knuckle-cracking sound. The most noteworthy technique was introduced in 2015 by researchers at the University of Alberta who concluded that cracking was caused by bubble formation. In their study, they had scanned fingers being pulled with an MRI to determine what happens to the synovial fluid inside knuckle joints. After a bunch of finger-pulling in the name of science, they identified, in the journal PLoS One, the cause of the noise as a bubble forming rapidly inside of the joint.
The findings were a strong lead, but Barakat noted that those imaging techniques could not provide the necessary time resolution to capture the high-speed dynamics of knuckle-cracking, and thus couldn’t determine whether collapsing bubbles might be behind the sound after all. So, Barakat, with his team, developed a mathematical model to rule out that option.
The new model considers three components: the change in pressure of the fluid as the knuckles move apart, the growth and collapse of the resulting bubble that forms, and the way changes in pressure turn into that ever-satisfying (or for some, agitating) cracking sound.
“What we demonstrate here is you don’t need full collapse,” Barakat said. His team found that even if a bubble only partially collapsed, it would still generate the loud sound, inferring that a separate process was behind the noise. Now we know for sure: The crack is not caused by the bubble bursting but by the bubble forming from the jolt of a newly opened space.
This conclusion may seem unintuitive. While we can all imagine the sound of a soap bubble delicately popping, it’s not as easy to imagine why the formation of a bubble might make a sound. In an interview with the Guardian in 2015, the author of the PLoS One paper, Greg Kawchuk, Ph.D., professor of rehabilitation medicine at the University of Alberta in Edmonton, explained how it happens:
If you’ve ever washed up glass plates, you’ll know they can be hard to separate when they are wet. 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.”
Even if we’re not bursting bubbles, there’s still a strategy for people to get the loudest crack out of their knuckles. Barakat’s team offers a few tips, citing speed as the most important factor. “The more rapidly you pull on your knuckle, the faster you are changing the pressure, and therefore the more likely you are to generate a knuckle crack.”