A computer simulation could finally explain why cracking the knuckles makes that awful sound

During the last decades, scientists have debated what is the true cause behind the sound or “pop” made by the knuckles when crunching them. Using computer simulations (or simulation models), a team of researchers from France could have found the answer.

According to the researchers explain in their study, published in Scientific Reports , the sound of the knuckles when crunching is caused by a “bubble that collapses in the synovial fluid of the joints matacarpofalángicas during an articulation release”. In other words, it is the sound produced by microscopic gas bubbles that burst within the joints of the fingers. Other scientists had proposed this theory for the first time almost 50 years ago, but the new study used a combination of laboratory experiments and a computer simulation to give more strength to the theory.

It may seem strange, but scientists have been investigating this body peculiarity since the early twentieth century and have not yet managed to reach a consensus on what is the cause of sound. The debate is the result of unconvincing evidence obtained in experiments and the difficulty of seeing the process as it happens, given that the whole phenomenon takes place in only about 300 milliseconds. However, scientists have agreed that not all people are able to crack their knuckles, not all fingers produce the sound of “pop” and that they have to spend around 20 minutes so you can crunch them again .

To understand the process better, researchers V. Chandran Suja and Abdul Bakarat, of the École Polytechnique de France, used geometrical representations of the matacarpofalángicas joints in which the small “burst” occurs, and converted them into mathematical equations to create simulations by computer from the process of crunching the knuckles. That is, computer simulations that reveal what happens on our fingers just before listening to the “pop”.

“Mathematical modeling is very useful because creating [real-time] images is not fast enough to capture this phenomenon,” Bakarat told Gizmodo. “Another advantage of modeling is that it allows you to vary one parameter at a time and, therefore, determine which parameters are really important to understand the behavior. In this way we find that the parameter that has the most effect on the sound generated when the fingers crunch is the strength with which you pull the knuckle. However, how fast you crack it, the geometry of the knuckle and the viscosity of the fluid (which changes with age) do not have a great effect. “The models revealed that when the joint is subjected to a certain amount of stress, pressure changes in the joint fluid cause the collapse of the microscopic gas bubbles within the synovial fluid. This theory was first proposed by scientists at the University of Leeds , in England, in the year 1971. But in 2015 a study published in PLoS One by researcher Greg Kawchuk, from the Faculty of Medicine of Rehabilitation of the University from Alberta, in Canada, used magnetic resonance imaging to reveal that gas bubbles remain in the fluid even after crunching the knuckles. According to the Kawchuk team, the sound is not caused by bubbles that explode but by bubbles that suddenly change their size.

But as revealed by Suja and Bakarat, this contradiction does not belie their theory. According to their models, only some bubbles need to burst to produce the “pop” sound, which explains why you can still see bubbles after cracking the knuckle. To test it, the researchers recorded the sound of the knuckles creaking from three test subjects, and compared their acoustic waves with the waves produced mathematically by the simulation. The result is that they are extremely similar, suggesting that the Suja and Bakarat model is an accurate representation of the knuckles when crunching, and that the bubbles exploding themselves are the cause of the sound.

Regarding the limitations of the study, Bakarat said that his team made a series of assumptions, including the presence of a single bubble, that the bubble is perfectly spherical, that the joint has an ideal shape or a common shape, and more. “A real limitation of the study is that we do not model the formation of the bubble in the synovial fluid, only its burst,” he said. “This work could, in the future, extend the modeling to include the formation phase of the bubble.”

Greg Kawchuk, lead author of the 2015 study, said that Suja and Bakarat “should be congratulated” for designing a mathematical model of a theoretical bubble. The researcher believes it is interesting that another phenomenon could be involved in the sound beyond his own research, but he also believes that the new study does not completely solve the mystery of blowing his knuckles.

“First of all I must emphasize that the work presented in this study is a mathematical model that has not been validated by physical experimentation. We do not know if this is what happens in real life, “Kawchuk told Gizmodo . “Secondly, although the authors of the study showed that the theoretical sound produced by a theoretical bubble was similar to the actual sound produced by the knuckles when they creaked, they did not prove the opposite circumstances when asking: what acoustics could be generated from the formation of bubbles? ? ”

And this is a good point, one that Bakarat himself recognized as a limitation of his research. Perhaps the very rapid formation of bubbles could produce a similar sound, but the studio did not consider it.”The impact of this new study is diminished by having investigated only one possibility (the collapse of an already formed bubble) without taking into account other phenomena, such as bubble formation or multiple collapse, as well as the emission of large volumes of gas in the joint after the sound that has already been visualized by many researchers, “said Kawchuk.

This topic may seem trivial, as Kawchuk comments, but the researcher believes it has potential for medical attention , since it could reveal new ideas to preserve joint health and joint mobility due to disease and aging.

As for whether knuckles are bad for health , the new study does not talk about that (and neither Bakarat nor Kawchuk wanted to give their opinion). But in 2015, Robert D. Boutin of the University of California, published an investigation that revealed that the habit does not produce immediately pain, swelling or disability in those who usually blow their knuckles, or in those who rarely do. Boutin also commented that “more research is needed to evaluate any long-term risk, or benefit, of blowing your knuckles.”

So if you enjoy blowing your knuckles you probably should not worry about getting arthritis or anything like that, just keep in mind that many of those who do not crack their knuckles consider that disgusting habit. Please, stop doing it.

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