Most people tend to be at one or the other extreme when it comes to knuckle cracking: either they find it enjoyable and/ or soothing, or the sound makes them want to jump out of their skin. Scientists still don’t know exactly why knuckles make that popping sound when they’re cracked, but believe it or not, they’re still trying to figure it out. What might be even more interesting than that is that more than one team has been working on a theory for knuckle cracking for decades.
The old theory for knuckle cracking
Scientists originally theorized that the popping sound that’s made when people crack their knuckles was due to the popping of a tiny bubble inside the joint, and one team of researchers believes they’ve found evidence to back up that theory. Researchers from Ecole Polytechnique in Paris and Stanford University in the U.S. created a mathematical formula they say supports the original theory, and then they published it in the journal Scientific Reports.
Joints are very complex structures, especially the small joints of the fingers and hand. Every joint in the human body has a cushion of synovial fluid lining it, enabling the bones to float without bumping into each other constantly. Sometimes tiny bubbles of empty space form within the joints, and Ecole Polytechnique Professor Abdul Barakat explained in their report that the sound created during knuckle cracking is actually due to a partial collapse of one of these bubbles. The knuckle cracking motion generally involves pulling the bones of the fingers apart, which briefly reduces the pressure inside the joint, making a popping sound as the bubble in the joint pops, the researchers said.
To determine what was actually causing the sound when people crack their knuckles, the researchers built a mathematical model to show what happens inside a joint while it’s being cracked, and they say that the formula they built backs up the bubble popping theory.
Another theory involving bubbles
One of the reasons scientists continued to look for a new theory about knuckle cracking stems from another study which found that there were still bubbles in the synovial fluid after the joints were cracked. Because there were still bubbles left in the fluid, scientists started looking at the problem from the opposite side, suggesting that the popping sound came from the formation of bubbles rather than from popping them.
University of Alberta researcher Dr. Greg Kawchuk led a 2015 study which brought about that theory. They used MRIs to take images of what’s happening inside a joint while it’s being cracked. In fact, the majority of the studies that have been done on knuckle cracking have used imaging-based methods, which is why the researchers from Stanford and Ecole Polytechnique decided to go the mathematical route instead. Their model seems to explain why there are still bubbles left in the synovial fluid after someone cracks their knuckles because they found that the popping sound could still occur with a partial collapse of a bubble.
Scientists will probably continue to look for reasons for that popping sound that’s made during knuckle cracking, although we already have some interesting tidbits of information from past studies. For example, according to Science X, other researchers have found that not all of the joints in the body are capable of being cracked, and it takes about 20 minutes to be able to crack a joint that was just cracked.