Why the awful sound is produced by knuckle cracking?

The sound produced by knuckle cracking is caused by the collapse of tiny bubbles.
The sound produced by knuckle cracking is caused by the collapse of tiny bubbles.

One of the most commonly found bodily phenomena is knuckle cracking. However, the phenomenon has puzzled not only the general population but also the scientists for over a century or even a more. Of course, it is not a big study that was published with a winning strategy, but it could help in solving the questions in the minds of many people. Nevertheless, in a recent study published in the journal Scientific Reports, it has been reported that the sound produced by knuckle cracking is caused by the collapse of tiny bubbles, in fact microscopic bubbles that are present in the joint fluid. 

In the study, researchers from the Ecole Polytechnique in Palaiseau, France, and Stanford University in California, U.S.A., utilized a mathematical model while considering the geometrical representation of the joints simulating the overall event of knuckle cracking.

“The sound that is generated when one cracks his or her knuckles is due to the partial collapse of a cavitation bubble that’s in the fluid (which is also known as Synovial fluid) in the joint,” noted Abdul Barakat, a professor at the Ecole Polytechnique and a coauthor of the study. Some researchers have proposed that the bubbles are formed from carbon dioxide and/or other gases.

“It could be multiple bubbles, but we showed that the collapse of a single bubble is sufficient to give you the signature sound you get,” he told AFP (a news source) in a telephonic interview.

In the year 1971, researchers initially proposed that the sound produced as a result of knuckle cracking could be attributed to the collapse of bubbles. However, their findings were debated by other researchers and studies, which was showing that bubbles were still there in the fluid after the cracking of knuckles. However, in the present study researches noted the production of sound as a result of partial collapse that could also help in resolving the apparent contradiction between the previous studies. In some of the earlier studies, ultrasound had also been used that showed the production of sound as a result of changes in pressure that is developed in the joint fluid. In a study published in the year 2015, after examining the knuckle cracking in a magnetic resonance imaging (MRI) scanning, it was also suggested that the sound is produced as a result of the formation of bubbles rather than their collapse. However, the imaging techniques would not be able to provide sufficient information related to high-speed dynamics of the cracking of knuckles. Therefore, in the present study, mathematical model is used and researchers worked on the simulation of knuckle cracking.

“We wanted to look at it mathematically because all the previous work was based on observation or imaging, so we tried to build a mathematical model that described the physical phenomena that governed this,” Barakat said. The mathematical model of the researchers had three important components: alteration in the pressure of the fluid as a result of moving apart of the knuckles, reproduction and collapse of the bubbles that are produced as a result the phenomenon, and the production of sounds from the bubbles as a result of changes in pressure.

“We showed that the collapse gave you the right signature sound.”

“The success of this relatively simple mathematical model fulfils two goals relevant to the current debate surrounding the origin of the sounds accompanying knuckle cracking. Firstly, the good correlation between the simulations and the experiments establishes support for cavitation bubble collapse as a potential source of the cracking sound. Secondly, the success of the model underscores the potential of detailed numerical simulations in resolving the origin of the sounds,” the researchers concluded in the study.

Although the study on the knuckle cracking would not completely help in resolving the contradiction but it would certainly start a series of knuckle popping as also noted by the researcher.

“After publishing this paper, my daughter tried it out and now she cracks her knuckles,” said Barakat.

Eventually, it is also important to note that knuckle cracking would not result in any kind of beneficial or harmful results as, for example, it would not result in arthritis. It is also important to note that not all the joints are able to make the characteristic sound, and the joints that can make the sound can only make after 20 minutes or so of initial cracking.

Usman Zafar Paracha

Usman Zafar Paracha is Assistant Professor, Pharmaceutics, in Hajvery University, Lahore, Pakistan.