Why do you hear popping when cracking your knuckles? Canadian scientists explain

WATCH ABOVE: U of A researchers have cracked the code on this medical mystery. Su-Ling Goh explains.

You crack your knuckles before taking on a big task, or maybe to loosen up after a tense day. Either way, what is that distinct popping sound when your knuckles crack?

To solve the conundrum, Canadian researchers used MRI technology to record what happens inside finger joints when people crack their knuckles. They suggest that what you’re hearing is a cavity forming inside the joint.

“We call it the ‘pull my finger study’ – and actually pulled on someone’s finger and filmed what happens in the MRI. When you do that, you can actually see very clearly what is happening inside the joints,” Dr. Greg Kawchuk, a professor at the University of Alberta, said.

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Researchers debated what causes joint cracking for decades: in 1947, British scientists first guessed that vapour bubble formations were the culprit. By the 1970s, other studies pointed to collapsing bubble formations.

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To settle the dispute, the Edmonton-based scientists collaborated with international peers in Australia to document what happens inside finger joints. A Nanaimo, B.C.-based chiropractor, Jerome Fryer, came up with the idea – he also became the resident knuckle-cracker in the quirky study.

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His peers dubbed him the “Wayne Gretzky of knuckle cracking.” Fryer had to insert his fingers, one at a time, into a tube connected to a cable that slowly pulled until the knuckle cracked as MRI video captured the movement in real time.

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Each time, the cracking and joint separation was quickly followed up with the creation of a gas-filled cavity. This opening was inside the synovial fluid, what the researchers describe as a “super-slippery substance” that keeps the joints lubricated.

WATCH: A University of Alberta research team led by Rehab Med’s Greg Kawchuk used MRI video to determine why joints make a popping sound when they crack.

“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,” Kawchuk explained.

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The team said its findings will open the doors to studying the benefits and harms of joint cracking. Past research has already warned that the amount of force needed to crack your knuckles is enough to cause damage to hard surfaces. On the other hand, studies haven’t found long-term harms to knuckle-cracking as a habit.

Kawchuk says his next steps are to come to a conclusion of his own when it comes to the safety of cracking knuckles. The findings could have implications for other joints in the body, such as the spine.

For now, his new study was published Wednesday afternoon in the journal PLoS One.

Read the full findings here.

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