Scientists offer new explanation for knuckle-cracking

The noise may emanate when bubbles in a joint’s fluid partially collapse

Stretching out your knuckles can sometimes release a sharp pop or two. Scientists have a new explanation for what’s behind the cracking.

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“Pop!” goes the knuckle. Why? Scientists disagree over what’s behind the cracking. Now, a new explanation comes from math. It suggests the sound results from the partial collapse of tiny gas bubbles in the joints’ fluid.

A role for bubbles is not new. Most explanations for knuckle cracking involve them. Those bubbles form under the low pressures produced by finger movements that separate the joint. Some studies have proposed that a bubble’s implosion creates the crack. But a paper in 2015 showed that the bubbles don’t fully implode. Instead, they persist in the joints for as much as 20 minutes after cracking. And that has suggested the bubble’s collapse doesn’t make the noise. So maybe its formation does.

But it wasn’t clear how a bubble’s debut could make sounds loud enough to be heard across a room. So two engineers from Stanford University and École Polytechnique in Palaiseau, France, took another crack at solving the mystery.

They now propose that the sound may come from bubbles that collapse only partway. The pair describe their reasoning March 29 in Scientific Reports. Using math, they simulated a partial bubble collapse. And it explained both the dominant frequency of the sound and its volume. That would also explain why bubbles can stick around long after a knuckle pops.

Physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

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