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What Causes That Popping Sound When We Crack Our Knuckles?

A new study dubbed the 'pull my finger study' finds out.

Have you ever wondered why your knuckles make that (oh-so-satisfying) "pop" or "crack"? Or, if the habit is harmless or harmful? Researchers from the University of Alberta did.

So, for a recent study, a volunteer's (one of the researchers who reportedly has amazing knuckle-cracking abilities) finger was connected to a cable and placed in a tube. The cable gently pulled his finger (we know, "ha ha") until his knuckle joint cracked. An MRI video captured each crack, confirming the tell-tale popping sound is caused by a gas-filled cavity. 

Science Daily reports: “The cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid, a super-slippery substance that lubricates the joints," says study author Greg Kawchuk. "It's a little bit like forming a vacuum. As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what's associated with the sound."

How's that for some water cooler chatter this afternoon?

The study is the starting point to figuring out if knuckle cracking really is helpful or harmful and researchers plan to investigate that next. Regardless of the answer, we bet you'll think of this the next time someone tells you to pull their finger. 

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