Nearly everyone has been annoyed by (or indulged in) articular release of the metacarpophalangeal joint—aka, knuckle cracking. Now, per the New York Times, one theory has risen over a competing one in the decades-long mystery of what creates the popping sound. In a study in Scientific Reports, journal researchers from France's Ecole Polytechnique created a multi-equation mathematical model (the BBC explains the equations) that suggests the popping sound is caused by the collapse of a gas bubble in the space between the knuckle bones. A theory from 2015 had said the sound was caused by the bubble forming, not collapsing, when the pressure dropped in the middle of the joint as the finger was pulled. Interestingly, the new study doesn't completely disregard the older study's conclusion, but perhaps complements it.
The researchers created their model of a joint with a bubble in it, then compared the predicted acoustic waves generated by the bubble collapsing with the sound of three volunteers cracking their knuckles. The sounds more or less synced up. So how does this jibe with the 2015 study, which showed a continued bulge in the knuckle (ostensibly the gas bubble) even after the knuckle had been cracked? The researchers say the sound can be produced by just a partial bubble collapse, leaving the rest of the bubble intact. The next step, the scientists say: to build a model showing the whole process, from bubble formation to collapse. Other tidbits: Once a knuckle has been cracked, it won't be able to make that same popping sound again for about 20 minutes. And if you can't crack your knuckles at all? There's too large a space between knuckle bones. (More on the 2015 study.)