Scientists Have Finally Figured Out What's Happening Inside Our Bodies When We Crack Our Knuckles

Thursday, 29 March 2018 - 1:03PM
Thursday, 29 March 2018 - 1:03PM
Scientists Have Finally Figured Out What's Happening Inside Our Bodies When We Crack Our Knuckles
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Image credit: YouTube
There are two kinds of people on this planet. Some people thoroughly enjoy the cathartic experience of popping or cracking their knuckles, enjoying the sensation of flexing their muscles to the point that their joints make an audible noise.

There are also people who think that this noise is the most horrid, abhorent sensation in all of the known universe.

There's a lot of misinformation floating around surrounding exactly what causes the cracking noise, and its effects on the human body. Some will claim, generally without scientific evidence, that popping knuckles can lead to arthritis in later life. Medical professionals object to this claim, but without a long-term study with a large participant base, it's hard to know for sure what the effects of habitual cracking might be.

Either way, scientists have finally been able to prove the origin of this noise, and it turns out we actually knew the answer all along.

All the way back in 1971, a study concluded that cracking the knuckles generated noise because air bubbles within the fluid in a person's joints pop when the joints are stretched. It all seemed logical—until a subsequent study in 2015 called everything into question.

Using ultrasound, a team of scientists proved that these bubbles are still present in the joints even after a knuckle has been popped. Considering that a cracked knuckle can't be re-cracked for around half an hour, it didn't make sense that these bubbles would both survive the joint stretching, and, for some reason, refuse to pop when again subjected to stress.

Thus, an alternative theory gained prominence: instead of the noise made by popping bubbles, this audible crack surely must be created by the formation of the bubbles!

Apparently not. Scientists from Paris' Ecole Polytechnique and Stanford University, eager to figure out what goes on during a knuckle pop, created a computer model to simulate how the bubbles must form in the fluid, and what must happen to cause them to pop.

As it turns out, the bubbles to pop during the stretching of the joints; they just don't collapse entirely, leaving some air still floating around in the fluid.

According to Abdul Barakat, a professor at Ecole Polytechnique:

Opening quote
"The sound that is generated when one cracks his or her knuckles is due to the partial collapse of a cavitation bubble that's in the fluid in the joint. It could be multiple bubbles, but we showed that the collapse of a single bubble is sufficient to give you the signature sound you get.
"We wanted to look at it mathematically because all the previous work was based on observation or imaging, so we tried to build a mathematical model that described the physical phenomena that governed this," Barakat said. We showed that the collapse gave you the right signature sound."
Closing quote

Of course, a smart as it might be to try to solve this issue with a computer simulation, this information is only useful if all of the data that went into the model is completely accurate.

Nevertheless, this feels like a success, as science is now able to answer a question that humans have been puzzling over for decades.

Now, someone just needs to use math to solve the bigger question surrounding popped knuckles: how to stop someone from getting grossed out by the noise.