Knuckle cracking

The man who had his knuckles popped for the experiment was the study's co-author, Jerome Fryer, whose fingers were placed in sheaths before being carefully pulled by a rope held by co-author Greg Kawchuck.

Fryer's digits were then slowly distended until the knuckle joints popped - all inside a rapid acquisition MRI.

"The MRI is a very loud environment, and I didn't want to pull his fingers off, so we had worked out this series of signals," said Kawchuck, a professor at the University of Alberta's faculty of rehabilitation, who added that this wasn't the most high-tech approach he'd ever used for an experiment.

The MRI he used, however, became the most advanced instrument to have ever observed a popping knuckle. The video it produced shows an air cavity forming just before the separation of the two bone surfaces, developing in the joint's synovial fluid.

While the researchers cannot say with certainty that the formation of the cavity is also the cause of the sound, since they could not place a microphone in the magnetic environment of an MRI, what they can say is this: The "vacuum" occurs at the same time the joints separate.

End of a debate?

It has also led the two researchers to take sides in the long-standing debate about what causes the sound of a knuckle popping.

In 1947, scientists posited that the noise had its origins from the formation of a gas cavity; a 1971 study, meanwhile, argued it was the collapse of that bubble of air which in fact caused the noise.

"This supports the 1947 idea, that it's the production of the cavity that's related to that sound," Kawchuck said, adding, "We now have to go forward and do some other experiments."

In addition to solving the conundrum of the popping sound, the professor, who also holds a research council chair at the World Federation of Chiropractics, hopes to shed light on the larger questions of knuckle cracking. For example: Why can some people do it, and others not?

"Is it something that you just have to learn, like whistling? Or is there something different about their joints?"

The current study can be found online in the peer-reviewed science journal "PLOS ONE."