The Enduring Mystery Of The Sound Of Knuckle Cracking May Have Just Been Solved

Are you a knuckle cracker? If so, good – it’s satisfying, it annoys people nearby who are oddly averse to this weirdly enjoyable action and, as highlighed by an Ig Nobel prize, it probably doesn’t do you any harm. Precisely why your knuckles crack, though, has remained something of an enduring mystery for decades.

A new study, courtesy of a pair of maverick researchers from Palaiseau’s École Polytechnique and Stanford University, has taken a rather unorthodox route to lending its support to one of those two leading theories. Published in Scientific Reports, they come down firmly on the side of one, while even addressing the quibbles academics have had with it.

Ladies and gentlemen, boys and girls: that popping noise is almost certainly the sound of the collapse of a bubble of synovial fluid inside your joints. Far from using experimental studies involving the pulling of real-world fingers, however, this study relied on the beauty of mathematics to present its case.

Between your joints, including your metacarpophalangeal (MCP) joints – your knuckles, basically – you have synovial fluid. This white-hued viscous matter is primarily designed to reduce the friction and subsequent erosion of the articular cartilage of said joints.

This fluid contains dissolved gases. Stretch them, and the space between your MCP joints increase, reducing the local pressure. This allows these dissolved gases to form microscopic bubbles, and if you leave your joints alone, they quickly redissolve back into the fluid, and you can crack away once more.

This is where the bemusement comes in.

The new study begins by noting that “despite over sixty years of research, the source of the knuckle cracking sound continues to be debated.” Essentially, studies that use MRI scanners or the like to peer at people’s cracking knuckles don’t capture all the fine details, and thus don’t provide enough conclusive information.

Since the 1970s, it was widely thought that the collapse of these air bubbles causes the audible “pop” sound. Fluid rushes in to fill the space you’ve made, crushing these bubbles and allowing some ephemeral, larger ones to form.

Sensible enough, but several recent papers involving the mechanized pulling of fingers concluded it was the formation, not destruction, of these bubbles that made the cracking noises. In fact, two papers giving opposing views were released in 2015. So which is it?

According to this new study, it’s the former. Eschewing physical observation, a mathematical model focusing on fluid dynamics was conjured up, and it found that the simulated bubble formation’s “acoustic signature” was consistent with both the magnitude and common frequency of popping heard in experimental measurements, both pre-existing ones and the pair’s own, which involved acoustic MCP donations from three volunteers.

Curiously, co-author Dr Abdul Bakarat, of École Polytechnique, told IFLScience that the “parameter to which the results are most sensitive is how hard you pull on the joint.” The best pop comes with the most effort, then.

One of the major problems with this model, however, is that recent experiments have shown that bubbles sometimes persist in the fluid long after the pop. If it’s their collapse that generates the pop, why would they linger around after the fact?

These two mathemagicians tell us not to worry. Their model shows that only a partial bubble collapse is required to create the sound, so these microbubbles are just harmless extras of the aftermath.

So, have they officially cracked the case? Possibly, but it’s worth remembering that – to quote Monty Pythonit’s only a model. Well-designed models can provide fantastic lines of evidence, but they can be wrong.

“I don’t think this study fully settles the score,” Bakarat added. Besides, they’ve only modeled the bubble collapse for now – modeling the formation is yet to come, and could provide a new twist in this most tantalizing of tales.

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