Cracking knuckles: You know the sound, but do you know what's behind the sound? Our contributor reviews the history of research on this fascinating topic.

© Raquelsfranca | Dreamstime.com

Our long period of waiting is over. Scientists, at last, have discovered what causes that popping, snapping sound of cracking knuckles.

So go ahead, America, crack your knuckles if you must. It will still be annoying to those around you, but—here’s the good news—it won’t damage your joints and it won’t lead to arthritis.

In spite of these breakthroughs, the research community is still puzzled as to why some people feel the need to crack their knuckles.

A Brief but Gripping History of Knuckle-Cracking Research

Scientists (hopefully in their spare time) have studied, published papers, disagreed, and changed their minds several times over the past 100 years regarding the mechanisms inside our knuckle joints that make a cracking sound. Take a look at the timeline, complete with various theories and the year they were put forth:

• 1939: Tightening capsule. Researchers at Stanford—yes, that Stanford, according to Time Magazine—published a review of knuckle cracking literature and theorized that the noise came from a tightening of the capsule surrounding knuckle joints.
• 1947: Vibrating tissues. Eight years later, scientists in England said the cracking-knuckles noise was caused by vibrating tissues when joints are forced beyond their normal range.
• 1971: Tiny bubbles. Yet another possible cause surfaced at England’s University of Leeds. Researchers suggested that the sound of cracking knuckles were caused by the formation of bubbles in the joint. They were on the right track.
• 2015: Gas-filled cavity. Canadian and Australian scientists discovered that even after a person cracks his or her knuckles, those bubbles remain. How could they make a noise if they haven’t popped? The researchers theorized in PLOS ONE that the cracking sound was associated with “rapid creation of a gas-filled cavity within the synovial fluid.” Some bubbles remain after knuckle cracking. Case closed? We wish.
• 2018: Collapsing bubbles. So now we’ve arrived at 2018 with the latest, and hopefully last, study on why cracking knuckles make that cringe-producing noise. In a study published in Scientific Reports, researchers developed a mathematical model showing that collapsing bubbles in the joint—not the formation of bubbles—are responsible for that distinctive sound. The sound can be produced even when a partial collapse of the bubbles occurs. A rapidly collapsing bubble—we’re not making this up—can produce a noise that reaches 83 decibels. That’s about as loud as a diesel truck traveling at 40 mph heard at a distance of 50 feet.

That settles it. For now.

Can Knuckle Cracking Damage the Joints?

No, cracking your knuckles does not appear to be harmful, at least in the short term, and it does not lead to arthritis, according to a study published in The Journal of the American Board of Family Medicine. Researchers used case studies of 215 subjects to determine whether an association existed between osteoarthritis and knuckle cracking. It did not appear to be a risk factor.

Habitual knuckle-cracking over a period of many years may, however, cause loss of grip strength.

A Few Final Knuckle-Cracking Facts

In case you just can’t get enough information about cracking knuckles, here are five more facts.

• Up to 54 percent of people crack their knuckles.
• Men crack their knuckles more than women.
• After cracking a knuckle, it takes 15 to 30 minutes to do it again. Bubbles need time to re-group.
• At least two reports showed that injuries occurred when people cracked their knuckles.
• Knuckle cracking accompanied by pain could indicate an underlying bone or joint abnormality.