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Scientists have figured out how knuckle-cracking works

Christophe Haubursin is a senior producer for the Vox video team. Since joining the team in 2016, he has produced for Vox’s YouTube channel and Emmy-nominated shows Glad You Asked and Explained.

After more than a half-century of clashing ideas, scientists think they've figured out what makes joints pop. In a study published Wednesday in the journal Public Library of Science, University of Alberta researchers claim that the cracking sound is a result of the formation of a bubble in joint fluid — not the sound of the bubble bursting.

Why hadn't we figured this out yet?

Debate on the topic has gone back and forth. The first study explaining joint cracking was published in 1947, when scientists used sequential X-rays to link popping to the creation of a bubble in joint fluid. That interpretation stood until 1971, when another group used a similar process to show that joint cracking may have been caused by the collapse of the bubble instead.

Co-authors Jerome Fryer and Greg Kawchuk now believe that the older hypothesis — that the popping sound is the creation of the bubble — was correct.

MRI knuckle

(Gregory Kawchuk, University of Alberta/PLOS Media)

Fryer — who was also the experiment's only subject in the study — lay in an MRI machine with tubes attached to his index finger. The tubes were pulled just enough to make the joint crack. After the pop, researchers noticed the creation of a bubble at the joint.

The project started when Fryer, a chiropractor, approached Kawchuk about the conflicting ideas on the topic. Fryer was the natural pick for testing, since he can pop all of his fingers on command.

Given that the study only tested one person, Kawchuk and Fryer still need to confirm their findings.

With further experiments, the results of this study could help us understand the effects of cracking joints; research has shown that habitual popping doesn't cause arthritis in the long term. The team plans to investigate why that might be, given their new understanding of how the process happens.

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