This sudden inflow of fluid is the popping you feel and what others hear when you crack your knuckles. The best guesses — a joint resettling or a tendon shifting. However, the most prominent guess was that it had to do with bubbles of carbon dioxide either forming or popping in the synovial fluid that lubricates and cushions joints.
The new study now confirms the latter theory. Led by radiologist Robert D. Boutin from the University of California, Davis, who presented it at the meeting of the Radiological Society of North America, the study is a first of its kind. Boutin oversaw researchers who recorded simultaneous audio and visual evidence of knuckles cracking. So, problem solved, right?
Not quite. All the investigators had done at this point was prove what the researchers already theorized. What about the dis proof—the later findings that the bubbles linger in the knuckle after the crack? The new model answers that one too: Yes, the bubbles collapse, but only partly. The sudden, if incomplete, contraction of the bubble is sufficient to produce an easily audible sound, while leaving enough bubble behind to be detected by MRIs.
The twenty minutes or so it takes for the next crack to be possible is the time it takes tribonucleation to create new bubbles and expand surviving ones. The final mystery—how a microscopic process can produce a sound that is so improbably loud—is actually the easiest one. Existing acoustic pressure equations show that given the speed of the bubble collapse and the environment in which it happens—the surrounding bones, the enclosing flesh—it is entirely possible to produce a crack that reaches 83 decibels.
That is roughly equivalent to the volume of a diesel truck moving at 40 mph, heard from a distance of 50 ft. Be respectful, keep it civil and stay on topic. We delete comments that violate our policy , which we encourage you to read. Discussion threads can be closed at any time at our discretion. Mystery solved: Why do knuckles crack? Michelle Starr. April 15, p. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Raymond Brodeur in the Ergonomics Research Laboratory at Michigan State University responds: To understand what happens when you "crack" your knuckles, or any other joint, first you need a little background about the nature of the joints of the body. Get smart.
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