Knuckle cracking

How it works

A joint is sealed in a capsule filled with synovial fluid, a slippery liquid that has gases dissolved in it². When you pull or bend a knuckle, the two bone surfaces move apart. The space inside grows, the pressure of the fluid drops, and the dissolved gas comes out of solution as a bubble. This is cavitation¹.

For decades nobody knew whether the pop was the bubble forming or the bubble collapsing. In 2015 a team at the University of Alberta put a volunteer’s finger inside an MRI scanner and filmed it being pulled. The sound came at the moment the bubble appeared¹. The bubble then stays in the joint. You cannot crack the same knuckle again for about 20 minutes, until the gas has dissolved back into the fluid¹.

The popular warning is that cracking causes arthritis. It does not. Donald Unger, a doctor in California, cracked the knuckles of his left hand every day for more than 50 years and never cracked the right. At the end, neither hand had arthritis. He published the result in 1998¹ and won the Ig Nobel Prize for medicine in 2009³. Larger studies comparing crackers with non-crackers have also found no link to arthritis¹. One older study reported weaker grip in habitual crackers⁴. A 2017 study of 40 adults found no difference in grip strength⁵.

How common it is

That 25% to 54% range comes from several studies, each of a different group of people⁴. About 25% of adults over 45 do it, 34% of 11-year-olds, and 54% of nursing home residents⁴. A 2011 study of 215 adults aged 50 to 89 found 20%, but it only included people who had X-rays of the hand⁴.

Try it

1. Make a loose fist with one hand.
2. Press the back of that fist with your other palm, bending the fingers a little further. Listen for a pop.
3. Or interlock your fingers, turn your palms outward, and push away from you.
4. Try the same knuckle again right away. Nothing happens. Wait about 20 minutes and it works again¹.

Is it a problem?

No. Knuckle cracking does not cause arthritis. Long-term studies, and Donald Unger’s famous decades-long self-experiment on his own hands, found no difference between cracked and uncracked joints¹³. An older study suggested weaker grip strength in habitual crackers, but later research did not confirm this⁴⁵. The popping sound and any looseness that follows are not signs of damage. See a doctor if a joint is painful, swollen, or stuck after popping, since that points to a different problem, not ordinary cracking.