If you’re a knuckle-cracker, you know how it goes: Sometimes you just don’t feel right until you pull, bend, or push your fingers until they pop and you’re flooded with satisfaction. On the other hand, if you’re not prone to cracking your knuckles, the urge might seem bizarre or even a little gross.
What causes that popping sound?
For decades, scientists have been trying to figure out what goes on physically to elicit that signature popping noise. A recent study in PLOS ONE has cracked, so to speak, the code behind this weird bodily function, revealing that the sound happens as a result of an air bubble that forms when a joint is pulled apart. The process is technically called “tribonucleation,” or the quick separation of two surfaces followed by a cavity formation, say the researchers in the study.
A team of University of Alberta researchers had a study participant place his fingers into a tubular finger trap one at a time. A cable attached to the finger’s tip then slowly pulled until a knuckle cracked.
The cracks were caught on MRI video so researchers could investigate what was going on, and each happened in the space of one frame (a.k.a. in 310 milliseconds).
While research from 1971 suggested the popping sound was due to the collapse of pre-existing bubbles in joints’ synovial fluid, this finding confirms a 1947 study that found it’s actually the creation of bubbles in the synovial fluid that causes the noise. Think of synovial fluid as the lubricant that exists between two joints. It’s necessary for proper joint and bone movement and comfort.
The joint separates, a dark bubble appears in the intervening fluid, and then everything settles back into place. Although the joint looks like it’s back to normal, it has to undergo a refractory period before it can crack again. “The fluid takes time to refill and create the same dynamics it had before,” says Michael Suk, M.D., chairman of the department of orthopedic surgery at Geisinger Health System. “It’s like pouring oil through a very small hole in an hourglass—it takes time for it to fill up again.”
Why do people even crack their knuckles in the first place?
“There’s both a mental and physical component,” says Suk. “From a mental standpoint, it’s almost a nervous habit for some people, much like drumming their fingers or biting their nails. I think to some degree, there’s a feeling associated with doing it as a mental stress reliever.”
Meanwhile, “from a physical standpoint, I think what this study shows is as you create space in the knuckle, you’re decompressing the joint,” says Suk. “In many cases, that can result in greater fluid movement in the joint itself, so your finger feels less constrained.”
Are some people just not capable of doing it?
Although it seems like some people can crack their knuckles without an issue and others can’t no matter what, that’s likely not the case. “If we understand joints to be what they are, everyone has the potential to crack their knuckles,” says Suk.
“The difference is that some people have a lower threshold of pressure for separating them, but others require much more force to create the separation.” Don’t take that as license to apply a ton of pressure just to hear the pop, though.
“There have been some reports that people can tear or stretch tendons based on how they crack their knuckles,”says Suk. “Some people pull, while others bend their fingers. Depending on how forcefully you do so, you can injure your hand.”
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