Instead, Barakat and Suja designed a two-dimensional mathematical model of the MCP joint to test the effects of different variables on the sound produced by collapsing cavitation bubbles, including the shape and geometry of the MCP joint, the force and speed used to pop the knuckles, and the viscosity of the joint fluid, which changes with age. That led the team to conclude that there are "pressure variations in the joint which causes the size of the bubbles to fluctuate extremely fast, and this leads to sound, which we associate with knuckle cracking", Suja tells the BBC.
After over a century of mystery, researchers have finally figured out what exactly causes the often-obnoxious cracking and popping sounds while someone cracks their knuckles. The verdict: the cracking sound was down to the rapid separation of the joint and bubble formation, not bubble collapse. Earlier research studies have actually developed that not all joints can be fractured, which- for that those that could- the act cannot be duplicated for one more 20 mins. "As you form this bubble you can cause pressure changes, and that can produce sound", said Barakat.
In the first study of its kind, researchers recorded simultaneous audio and visual evidence of knuckles cracking.
Scientists from the United States and France now believe they have a definitive answer to why the habit produces its unique pop, thanks to three new mathematical equations. Kawchuk suggests that a plausible next step would be to see whether the formation of the bubble in a similar model could create sound.
The third, according to Mr Suja, who is now a postgraduate student at Stanford University in California, was "coupling the size variation of the bubbles to ones that produce sounds". Barakat has an answer. What might be even more interesting than that is that more than one team has been working on a theory for knuckle cracking for decades.
"What we demonstrate here is you don't need full collapse", he said, pointing out that even if the bubble just partially collapsed to leave a micro-bubble, it would generate the sound on the necessary timescale.
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The authors of the newly published study didn't fully agree with the 2015 analysis, and think a mathematical model is more accurate than a MRI. "Their main finding, that theoretical bubble collapse can create sound, is not surprising", he said. Which might explain why some people have a harder time cracking their knuckles - their joints might simply be too wide.
The study also reveals why some people can't crack their knuckles.
While there has been some debate about whether knuckle-cracking increases the risk of osteoarthritis, studies do not appear to support a link. Dr. Donald Unger received an Ig Nobel Prize in 2009 after he experimented with cracking the knuckles in his left hand while leaving his right knuckle free from cracking.
"After publishing this paper, my daughter tried it out and now she cracks her knuckles", said Barakat, himself a knuckle-cracker.
"When we crack our knuckles we're actually pulling apart our joints and when we do that the pressure goes down".