Here’s the Actual Physics of Why Knuckles ‘Crack’

Mathematical models show that the trademark popping sound comes from the collapse of tiny cavitation bubbles in the knuckle joint.

Image Source via Getty Images
Image Source via Getty Images

Bored and anxious humans have long ‘cracked’ their knuckles. But only in the past 60 years have scientists put serious thought into the physics of knuckle-cracking. The reigning theory, proposed in 1971, was that the sharp popping sound was caused by the collapse of small cavitation bubbles in the metacarpophalangeal (MCP) joints, the third knuckle down from the tips of your fingers.

But a 2015 paper cast doubt on the bubble-collapse theory, using real-time MRI imaging to argue that the creation of cavitation bubbles, not their collapse, was the true cause of the trademark cracking sound.

Now a team of French scientists believe they’ve restored the reputation of the original bubble-collapse theory by using mathematical modeling to prove that only a rapidly collapsing cavitation bubble could generate acoustic waves powerful enough to produce the classic pop, according to a article published in the journal Scientific Reports.

Abdul Barakat, co-author of the paper, is a professor of biodynamics at the Ecole Polytechnique where he specializes in modeling cardiovascular mechanics to better understand heart disease. It was one of his master’s students, first author V. Chadran Suja, who came up with the idea of modeling the unsolved mystery of knuckle-cracking.

Barakat told Seeker that it’s the motion of separating the knuckles in the MCP joint that creates the cavitation bubbles, tiny cavities or "voids" of gas in the joint's synovial fluid.

“If you have two surfaces that are separated by a viscous fluid and you pull these surfaces apart rapidly, you depressurize the fluid in between the surfaces,” explained Barakat. “That sudden decrease in pressure can make it so that dissolved gases in the fluid can nucleate into bubbles.”

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The person cracking their knuckles doesn’t actually “pop” the cavitation bubbles by manipulating the joint. Instead, the unstable bubbles collapse on their own accord almost instantly, generating pressure waves that translate into sound.

One of the gripes from the 2015 paper was that the fraction of a second that it takes for the bubbles to fully collapse is too slow to produce the popping sound quickly enough. What Barakat and Suja’s mathematical models show is that even a partial collapse of a bubble was sufficient to generate the pressure waves, matching the speed and intensity of real-world cracks.

The French researchers chose mathematical modeling over the imaging technique used in the 2015 paper, which attempted to record MRI scans of internal processes in real time. Barakat and Suja believe that the imaging technique, called cineradiography, hasn’t yet achieved the necessary resolution (1,200 frames per second) that is necessary to see bubble collapses in action. The frame rate of the 2015 paper, for example, was only 3.2 frames per second.

Instead, Barakat and Suja designed a two-dimensional mathematical model of the MCP joint to test the effects of different variables on the sound produced by collapsing cavitation bubbles, including the shape and geometry of the MCP joint, the force and speed used to pop the knuckles, and the viscosity of the joint fluid, which changes with age.

What they found was that the distance separating the knuckles was one critical variable. Knuckles that are closer together in a resting position produce bigger bubbles when separated during the cracking motion. Which might explain why some people have a harder time cracking their knuckles — their joints might simply be too wide.

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But the biggest parameter driving knuckle-cracking turned out to be the force applied during the crack.

“For people who can’t pop their knuckles normally, I predict that they would be able to if they could generate enough force, but you don’t want to break your finger trying to crack your knuckles,” said Barakat.

The most convincing result was that Barakat and Suja’s bubble-collapsing model produced pressure and sound waves with nearly identical amplitudes across time as real-world knuckle-cracking experiments. In other words, the mathematical pops sounded exactly like real pops, at least on paper.