It’s one of the most annoying habits, but it's also pretty mysterious. What actually causes the sound of cracking knuckles?
Researchers long believed the sound was caused by the collapse of bubbles that form in the fluid-filled space of your knuckles, reports Science’s Katie Langin. But a 2015 study suggested that wasn't the case. Instead, the researchers claimed that the sound likely occurred when a fluid rushed into the cavity and the bubbles formed.
Now, new research puts scientists back where they started. In a recent study published in Scientific Reports, researchers used mathematical formulas to support the idea that the collapse of bubbles in the joints generates the cringe-inducing popping sound.
As Helen Briggs reports for BBC News, science student Vineeth Chandran Suja decided to look into the source of the sound when he found himself cracking his knuckles in class. Along with his professor, Abdul Barakat of École polytechnique, he came up with a series of three equations to model what's happening in the knuckles during cracking.
The mathematical model suggests that the collapse of bubbles—not their formation—causes the crackling. But they don't necessarily have to entirely collapse. As Briggs reports, tiny bubbles can remain.
Here’s what we know about the process: Knuckle are filled with synovial fluid that lubricates the joints. When the bones are pulled apart, as they are during cracking, a sudden drop in pressure in the joint causes the dissolved gases in the fluid (mostly carbon dioxide) to form bubbles, Veronique Greenwood explains for The New York Times.
Chandran Suja, now a postgraduate student at Stanford University, tells Briggs that their work suggests that pressure variations in the joints cause the size of bubbles to fluctuate, and this is what causes the sound we all know so well.
While the new study doesn’t totally settle the debate, it bridges the gap between the two previous conflicting theories. For the 2015 study, researchers demonstrated that bubbles remained in the fluid after cracking—evidence supporting the bubble-forming theory of cracking. But the new study suggests that the sound can be produced even when only a partial collapse of the bubbles occurs, Briggs reports.
But there are still some things we don’t know. The new research does not explain, for instance, what happens as the bubble forms; it only assumes that formation happens. A next step in the study of knuckle cracking could be to model the entire process from bubble formation to bubble collapse.
And what about the people who can’t crack their knuckles? The researchers also have an explanation for that. “Some people cannot crack their knuckles because the spacing between their knuckles is too large for this to happen,” Barakat tells The Guardian’s Nicola Davis.
So if you're one of the lucky (or unlucky) few that can create a good crackle, you can thank the tiny bubbles in your joints for that cringe-worthy sound.