Cracking knuckles: what makes the popping sound? – Medical News Bulletin

Health, Fitness & Food


Scientists in the United States and France have developed a mathematical model to explain the popping sound when knuckles crack.

If you have ever cracked your knuckles, you may have wondered what makes the popping sound. According to The Uniformed Services University of the Health Sciences, nearly 20% of people have a habit of cracking their knuckles. The reasons for some people include a nervous habit and seeking relief from stress. But what exactly makes the popping sound?

For many years scientists believed that the source of the popping sound came from the articular release in the joint. They believed that due to the pressure shift, gas bubbles would form and pop causing the noise when knuckles crack.

In a recent study published in Scientific Reports, a group of scientists in the United States and France developed a mathematical model to describe the events leading to the generation of the popping sound when knuckles crack. The model consists of a series of equations to explain the sound that accompanies the release of the joint between the fingers and the bones.

The first dynamic equation describes the room-temperature pressure variations of a collapsing cavitation bubble during an articular release. The second equation reconstructs the acoustic pressure field to recover the sound generated in response to the pressure variations. The third equation couples the size variation of the bubbles to ones that produce sounds. The developed model was then validated against the cracking sound recorded by an advanced microphone.

The model demonstrates that only a partial collapse of the bubble is needed to replicate the experimentally observed acoustic spectra. The result implies that the bubble can hang around in the joint fluid after the knuckle has been cracked. The findings may provide insight towards resolving the ongoing debate about the origin of what makes the sound when knuckles crack.

Written by Man-tik Choy, Ph.D

Reference: Chandran Suja V. & Barakat A. I. 2018. A Mathematical Model for the Sounds Produced by Knuckle Cracking. Scientific Reports, 8, 4600. DOI 10.1038/s41598-018-22664-4.



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