What Happens When You Crack Your Knuckles?

Maybe you've heard the words, "Don't crack your knuckles - you're going to give yourself arthritis!" Maybe it made you immediately stop in your tracks, or perhaps you just rolled your eyes but secretly wondered if your admonisher was right. Well, wonder no more - if you were in the latter category, you're in the clear. There has never been a case of arthritis found to be caused by knuckle cracking, even if you do it frequently.

But have you ever wondered what happens when you pop your knuckles? Bone isn't grinding on bone like many have thought - your knuckle actually cracks as a result of changing pressure in the fluid that lubricates the joints. The fluid has a thick, honey-like texture, and scientists still aren't sure if the popping noise means that a bubble is being formed in the fluid or if a bubble is literally being popped. Read on to discover the studies that have been done on this fascinating topic, and don't forget to send this to your mom if she always told you to quit cracking your knuckles!

Photo: orijinal / flickr / CC-BY 2.0

  • The fluid that surrounds your knuckles "pops" when there's a change in pressure.

    Radiologist Robert D. Boutin from the University of California, Davis, led a research team to investigate what exactly happens when a knuckle is cracked. Participants cracked the knuckle at the base of each finger, the metacarpophalangeal joint in particular (MPJ), while the finger was being observed through an ultrasound machine. They took images of a total of 400 MPJ cracks.

    Ultrasound images showed a visual "flash" when a knuckle was cracked. Researchers noticed a direct correlation between the popping sound and flash visual.

  • Nope, you're not cracking bones.

    As Boudin and his team of researchers proved, you are definitely not pushing your bones against each other when you crack your knuckles, despite what you might have heard from teachers or parents. The noise actually comes from a gas bubble trapped in the synovial fluid which surrounds your joints.

    "There have been several theories over the years and a fair amount of controversy about what’s happening in the joint when it cracks. We’re confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."  Boudin said.

  • We're still not sure exactly what is happening.

    While we know that the synovial fluid is involved in joint cracking, we're still not sure if the air bubbles in the fluid are collapsing or being formed when knuckles are cracked. 

    The Washington Post quotes Boudin as saying, "That's a surprisingly tough question to answer. I will tell you that we consistently saw the bright 'flash' in the joint only after we heard the audible crack. Never the other way around. Perhaps that supports the bubble formation theory, not the bubble popping theory."

  • Knuckle popping could actually increase joint range of motion.

    While Boudin did say more research needs to be conducted to confirm that no long-term damage is being done, there’s a real possibility that joint-cracking is actually good for you in the short term - specifically for your range of motion. He said, "After a joint cracks, the range of motion for that joint increases significantly."

  • No, it doesn't lead to arthritis... but it's probably best to crack in moderation.

    If you had a parental or authority figure get mad at you for popping your knuckles because "it could cause arthritis," they were probably just trying to be helpful - but there's zero evidence to support the belief that cracking your knuckles leads to arthritis. Yes, you read that right - zero evidence. It's basically an old wive's tale. So crack away! Just don't go too overboard - long-term effects of overzealous cracking have yet to be concretely determined.

    And those who pop frequently, according to Harvard Health, "were more likely to have swollen hands and reduced grip strength. And there are at least two published reports of injuries suffered while people were trying to crack their knuckles." So you're probably in the clear for long-term health effects, but moderation is best to be on the safe side.

  • Cracking too frequently can lead to inflammation.

    Anything involving moving tendons (which cracking your knuckles does) can cause inflammation if done frequently. And, according to a research study by Jorge Castellanos and David Axelrod called "Effect of habitual knuckle cracking on hand function," "Habitual knuckle cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol. It is concluded that habitual knuckle cracking results in functional hand impairment."

    So you're good to crack on - again, just try not to overdo it!