Bodily Oddities

Knuckle cracking

Also called: Joint cracking · Crepitus · Joint cavitation

Pulling or bending a finger joint until it pops. Between 25% and 54% of people do it out of habit. The sound is a gas bubble forming in the joint fluid, and it does not cause arthritis.

Bones and joints Trick Voluntary
Try it Debunked myth Record

How common

25–54% crack habitually Common · 10–50%

Can you do this?

These are answers from visitors, not a study.

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.

Between 25% and 54% of people crack their knuckles regularly1.

The popular warning is that cracking causes arthritis. It does not. Donald Unger, a doctor in California, cracked the knuckles of his left hand at least twice a day for more than 60 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 slightly weaker grip in habitual crackers. Later work did not confirm it1.

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 20 minutes and it works again.

Sources and further material

  1. Encyclopedia Cracking joints The cavitation mechanism, the refractory period, and the arthritis question. The 2015 MRI study was Kawchuk et al., University of Alberta.
  2. Encyclopedia Synovial fluid The fluid inside the joint that the bubble forms in.
  3. Encyclopedia Ig Nobel Prize Donald Unger won the 2009 prize for medicine for his one-handed self-experiment.
  4. Video Knuckle cracking MRI video YouTube search.
Bones Variation

Joint hypermobility

About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.

10–25% of people
Bones Trick

Voluntary joint dislocation

Pushing a joint partly or fully out of its socket on purpose, then back in. About 20% of people have some joint hypermobility. True voluntary dislocation is much rarer and often linked to a connective tissue disorder.

Rare; no formal count Icky
Ears Sound

Ear clicking when swallowing

Every swallow opens a small tube between your throat and your middle ear for a fraction of a second. The faint click you can hear is that tube letting air through to balance the pressure.

Everyone