Researchers pull fingers to solve why knuckles crack | Science News

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Researchers pull fingers to solve why knuckles crack

Finding bursts bubble of popular theory

2:00pm, April 15, 2015

KNUCKLING DOWN  A knuckle at rest (left) pops when pulled. The sound of the knuckle cracking comes from a bubble (right, arrow) opening in the joint.

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Knuckles crack when a bubble forms in a joint, new high-speed images reveal.

The finding, reported April 15 in PLOS ONE, may settle a decades-old debate about the source of the sound.

In 1947, two researchers used a series of X-rays to determine that the “Crack!” comes when joints rapidly separate to form an air bubble, a process called cavitation. A 1971 study used similar methods but concluded that the pop is the sound of the bubble bursting.

Gregory Kawchuk, a bioengineer and rehabilitation medicine specialist at the University of Alberta in Canada, and colleagues used faster, more detailed magnetic resonance imaging, or MRI, to watch what happened as they slowly pulled a man’s finger until his knuckle cracked. Coauthor Jerome Fryer, a chiropractor in Nanaimo, Canada, was the test subject. &ldquo

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