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Watch This Gross MRI of a Knuckle Cracking

• Wired.Com

Human beings—well, the rude ones—crack their joints. It's a thing. But scientists have never really understood the physics behind that chilling noise. (And yes, they care.) In the 1970s, most experts thought it had to do with the collapse of air bubbles in the synovial fluid that lubricates joints. But new evidence suggests the sound is actually caused by precisely the opposite: the formation of a gas-filled cavity when the bones in joints stretch apart.

But how would one study such a thing? First, researchers at the University of Alberta found someone who could crack his knuckles over and over again, without the long refractory period most people have. Yup, he was multiply crackasmic.

Then the scientists put this crack-addict's fingers into a magnetic resonance imager, watching cracking events as they took place. That's what's in the GIF we made you from the researchers' video. As the bones in the joint separate, negative pressure means gas (likely nitrogen) in the synovial fluid gathers together, resulting in the sudden formation of bubbles—the scientific term for that is tribonucleation. And with that comes the pop.

Real-time-MRI-of-joint-cracking


www.universityofreason.com/a/29887/KWADzukm