Why do your knuckles pop?

What’s that sound? Depending on whom you ask, the crackle of popping joints is either the sound of sweet relief or the noxious tones of a stomach-turning habit. Really, though. What’s that sound? I mean, why does bending your joints in a certain way make them pop like that? Scientists have offered several explanations, including rapidly stretching ligaments, and in severe cases, actual bones grinding against each other.

But the most common explanation for why your stretched-out joints sound like bubbles popping is that, well, there are bubbles in there. The joints in your fingers are the easiest ones to crack, but many people also crack the joints between vertebrae in their neck and back, and even their hips, wrists, shoulders and so on. All these joints are synovial joints, and they’re the most flexible ones in your body.

The space between the two bones is filled with a viscous liquid, synovial fluid, which contains long, lubricating molecules, like hyaluronic acid and lubricin. Synovial fluid is more or less the texture of egg yolk and its primary purpose is to cushion the bones and help them glide past each other. It also contains phagocytic cells that help clean up any bone or cartilage debris that ends up in the joint. But the reason it’s important for knuckle cracking is that, like other fluids in your body, it contains lots of dissolved gas molecules.

Knuckle-crackers know that to get that satisfying pop, you stretch the joint farther than it normally goes by bending your fingers backwards, for example. When you do that, the bones move away from each other. The space between bones gets bigger, but the amount of synovial fluid stays constant. That creates a low-pressure zone that pulls dissolved gases out of the synovial fluid, just like the carbon dioxide that fizzes out of soda when you twist open the cap. Inside the joint, the escaping gases form a bubble with a pop.

But the bubble doesn’t last long. The surrounding fluid presses on it until it finally collapses. The bubble’s gases scatter throughout the synovial cavity and slowly dissolve back into the fluid over the course of about twenty minutes, which is why it can take a while before you can pop the same joint again. Some scientists think there may actually be two pops. One when the bubble forms, and another when it bursts.

Popping a joint temporarily enlarges it, which may be why dedicated knuckle-, neck- and back-crackers say the habit makes their joints feel looser and more flexible. But you may have heard from a concerned relative or annoyed officemate that cracking your joints will give you arthritis.

A doctor named Donald Unger heard this, too. So, determined to disprove his mother’s warnings, he cracked the knuckles of his left hand repeatedly for 50 years, while the right-hand knuckles went unpopped. 36,500 cracks later, both hands were arthritis-free. For this selfless act of devotion to science, Dr. Unger received an Ig Nobel Prize, a parody of the Nobel Prize that recognizes wacky, but weirdly fascinating, scientific accomplishments.

Unger wrote that his results should prompt investigation into other parental beliefs, like the importance of eating spinach. The jury’s still out on that one. As for knuckle-cracking, one study suggests that all that joint stretching and bubble bursting can cause your hands to swell and weaken your grip. But the biggest proven danger seems to be annoying those around you.

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