Scientists Take a Crack at Explaining That Knuckle-Popping Noise

A new study takes a closer look to explain the mechanism behind the sound when we crack our joints

Image Source/Corbis

Knuckle-cracking is both commonplace and cryptic: Why on earth do our knuckles make that annoying popping noise when we crack them? Scientists have speculated on the question for decades, but only now have they finally found the mechanism behind the racket. All it took was an MRI, a knuckle-popping hotshot and someone to repeatedly pull his finger.

After meticulously analyzing the MRI footage of the subject’s knuckle being popped, the international group of researchers determined that the sound associated with the action emanated from the rapid formation of a cavity within the joint. “The crack apparently comes from a bubble forming in the fluid within the joint when the bones separate,” Rob Stein explains in NPR. “It’s a bit like a tiny airbag inflating.”

In order the carry the study out, the researchers needed someone who had a particular knack for cracking his or her joints. Fortunately, they knew the perfect candidate. “Most people have the ability to crack their knuckles, but unlike most, [Jerome] Fyer can do it in every finger, and after the standard recuperation time, he can do it again,” the press release says. In fact, “Fyer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team,” says Gregory Kawchuck, lead author of the study that was published on Wednesday in PLOS One.

The press release elaborates on the team’s process and results:

Fryer’s fingers were inserted one at a time into a tub connected to a cable that was slowly pulled until the knuckle joint cracked. MRI video captured each crack in real time – occurring less than 310 milliseconds.

In every instance, 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.

“It’s a bit like forming a vacuum,” Kawchuck said. “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 even is what’s associated with the sound.”

Here’s the MRI video of a knuckle crack in action. It might be cringe-inducing, but according to the researchers, popping your joints will not harm them. And knowing exactly what's making that noise could give you that much more joy with each crack of the finger.

Pull my finger: Why joints crack

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