The sound of popping knuckles has long been a source of bafflement for scientists. Now researchers say they might have cracked its origins.

While previous research has shown that not all joints can make the sound, and that those that do can only be cracked once every 20 minutes or so, quite what is behind the auditory pop has been a topic of hot debate.



“The cavity in between the two knuckles is filled with a fluid that is called the synovial fluid, and when you suddenly change the pressure in that fluid as a result of increasing the spacing between the knuckles, some of the gases in that fluid can nucleate into a bubble,” said Prof Abdul Barakat of the Ecole Polytechnique’s hydrodynamics laboratory, a coauthor of the new study.



Some researchers have suggested that it is the collapse of such bubbles, formed of carbon dioxide and other gases, that causes the well-known crack, but others have proposed another possibility. “As you form this bubble you can cause pressure changes, and that can produce sound,” said Barakat.

In 2015 researchers in Canada appeared to have solved the puzzle, after one of the team had his knuckles cracked in an MRI scanner as images were taken. The verdict: the cracking sound was down to the rapid separation of the joint and bubble formation, not bubble collapse.



Barakat says the idea of delving deeper into the issue came from one of his students, a coauthor of the new research, who chose to study the phenomenon for a course project.

Noting that imaging techniques do not provide the necessary time resolution to capture the high-speed dynamics of knuckle-cracking, the pair developed a mathematical model to explore whether collapsing bubbles could be behind the sound after all.



The model, said Barakat, is based on three components: the change in pressure of the fluid as the knuckles move apart, the growth and collapse of the resulting bubble, and how changes in pressure from the bubble turn into sounds.

The team compared the sounds they would expect from collapsing bubbles produced from joint-popping, according to the model, with sound patterns recorded from a handful of knuckle-cracking participants, and found a good match between the two. By contrast, Barakat says formation of bubbles has not been shown to produce sounds of the observed magnitude or loudness.



But there is an extra nuance: some have argued that it takes longer for the bubble to collapse than for a crack to be heard, and that this makes it an unlikely source of the sound. Barakat has an answer.



“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. The discovery, the authors add, could explain why small bubbles have be observed in synovial fluid even after knuckle-cracking.



Dr Greg Kawchuk from the University of Alberta, a coauthor of the 2015 study, welcomed the new research. “Their main finding, that theoretical bubble collapse can create sound, is not surprising,” he said. “What makes this paper interesting is that it suggests that other phenomena may occur in between frames of the MRI video published in our prior study and that these phenomena may create sounds that are similar to those produced in knuckle-cracking.”

But, he added, the case was not yet closed, noting that the latest research is a mathematical model that has yet to be verified by experiment.

While there has been some debate about whether knuckle-cracking increases the risk of osteoarthritis, studies do not appear to support a link.

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Among those to study the phenomenon was Dr Donald Unger, who won an IgNobel Prize in 2009 at the age of 83 for cracking only his left knuckles since his teenage years, while leaving his right knuckles uncracked. Unger reported no signs of arthritis in either hand.

Not everyone can produce a knuckle crack. “Some people cannot crack their knuckles because the spacing between their knuckles is too large for this to happen,” said Barakat.

But, for those who can and enjoy the sensation, Barakat has a tip: “The more rapidly you pull on your knuckle, the faster you are changing the pressure and therefore the more likely you are to generate a knuckle crack.”