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What's actually happening when you crack your knuckles?

Bruno Vincent/Getty Images

There's a long-held myth that cracking your knuckles can give you arthritis. The sound definitely might cause the people around you to cringe, but what's making those noises and is it actually bad for you?

Is cracking your knuckles bad for you?

You might wince when you crack a joint, and you might also feel a release. If it hurts when you do it, then you should avoid it and see a doctor. But otherwise, is popping your knuckles (or your back or shoulders) bad for you? Probably not.

(Library of Congress)

Why do knuckles pop? Within joints, there's a space filled with something called synovial fluid that reduces the friction in your joints when you move. The fluid works like a lubricant and contains gases (oxygen, nitrogen, and carbon dioxide).

When you pop a joint, you stretch out that space between bones. That expanding space creates negative pressure, like a vacuum, that sucks the synovial fluid into the stretched-out space. It forms bubbles. And the bubbles collapse, which is what you hear.

In order to crack the same knuckle again, you have to wait about 20 minutes until the gases return to the fluid between the joint. The more often you pop a joint, the looser it gets. The looser it gets, the easier it is to pop in the future.

But is that looseness the first sign of impending trouble and future arthritis? Likely not. Consider the findings of Donald Unger, arguably the most dedicated researcher of knuckle cracking. In his teens, after his mother warned him the habit would give him arthritis, he started cracking his knuckles in the name of science, according to a 2009 Los Angeles Times story. He continued pop the joints in his fingers for over 60 years — but only on his left hand. He left the right hand untouched as a comparison. And his decades-long self experiment found no arthritis in either hand.

Although he's just one case study, most medical sources agree with Unger's finding that there's no link between popping your knuckles and arthritis. A 2010 study of 215 people found that a history of knuckle cracking isn't a risk factor for developing arthritis in the hand. (Surprisingly, those who didn't crack their knuckles had slightly higher rates of arthritis than those who did.) A 1975 small-scale study of patients in a nursing home also found no correlation.

One 1990 study of 300 people did find that cracking knuckles over a long period of time led to hand swelling and decreased grip strength, but there hasn't been any follow-up research on it.

The sound you're hearing might not be joint cracking at all

The sounds your back or knees can sometimes make not be your joint popping (Shutterstock)

Joint cracking can often be confused with the snapping sound tendons make when they slide between muscles or over bones. Tendons work like rubber bands stretched between muscles and bones to connect the two. When a joint moves, the tendon snaps quickly over it and can sometimes make a popping sound. It's common to hear these sounds in the knees and ankles when you go from sitting to standing, or vice versa, or when you're walking up or down stairs.

There's also another reason for joint popping, called crepitus. It's the grinding sound that you might hear when a bone moves against cartilage, a firm tissue used as a connector in many places in the body. Crepitus happens most often in the knee, is harmless, and is often described as sounding and feeling "crunchy."

You shouldn't worry about these snapping or crunching sounds unless they started happening after an injury or are accompanied by pain. They're just your body doing its thing.