Here’s why your knuckles make a popping sound when you crack them

People crack their knuckles to calm anxiety, relieve stiffness, prepare for an epic kung-fu fight, and annoy the hell out of their co-workers. Also, it feels good. At least, that’s what I’ve heard. I’ve never cracked my knuckles before, and I don’t plan to. That POP is pretty unsettling. Whenever I get nervous, I bite my fingernails, because that’s not gross at all.

But why do your knuckles make that popping sound when you crack them? Scientists have furiously debated this issue for years. All people knew for sure was that it didn’t sound healthy. That’s probably what inspired those old wives tales. We’ve all heard those rumors that cracking your knuckles might make them deformed, or lead to getting arthritis. But what’s the truth? How do joints so small make so loud a noise?


day #7 BAD HABBIT #crackingknuckles

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In 1971, researchers at the University of Leeds thought they solved the mystery. They hypothesized that the cracking sound came from the collapse of bubbles in the fluid of the joints. This theory makes sense, since a knuckle that’s cracked cannot be immediately re-cracked. But decades later, the idea was debunked, when researchers discovered that the bubbles remained in the joint fluid after the knuckles were cracked.

Now, a study in Scientific Reports finally offers an explanation. Researchers Vineeth Chandran Suja and Abdul Barakat created a mathematical model to figure out the original of the popping sound. By using a geometrical representation, a mathematical equation and other big important-sounding science words, they deduced how the noise is made.

Their conclusion? When you crack your knuckles, tiny bubbles of fluid in the joint do collapse, just like those hippie scientists in the 1970’s thought. However, their research uncovered one important detail: The bubbles only have to partially collapse to produce that pleasurable, yet annoying pop. This explains why those pesky microbubbles of fluid were still found in the joints after cracking.


But one question remains: How does the collapse of such tiny bubbles produce such a loud sound? It just does, man. The authors showed how the pressure released by the collapsing bubbles produced acoustic waves, which can be calculated by their mathematical model. Given the speed of the bubble collapse and the claustrophobic environment of bone and flesh, it’s possible to produce a crack that reaches 83 decibels. That’s loud enough to intimidate precisely three thugs before you get in a badass street fight.

It’s unclear if this research has a practical application, but hey, at least we understand the human body a little better. I’ll never become a knuckle-cracker, but there are times when I appreciate the sound. In 1993, the Wu-Tang Clan sampled the sound of cracking knuckles to create the beat for their classic song, “Bring The Ruckus.” That song is awesome.

In a related story, here are 18 facts about the human body that will majorly freak you out…