Cracking the mystery of the knuckle crack

Let me start by saying, constant knuckle cracking can make you look like a social hazard in a sane environment, so a good tip is to lose the habit.

There are quite a few theories on the story behind the sound. These include:

a) Broken adhesions: As two cartilage surfaces are pressed together, they form adhesions, and when the joints are separated this makes the popping or cracking sound.

b) Rapid stretching of ligaments.

c) Cavitation (The most popular one): Knuckles are surrounded by Synovial fluid (the fluid has about 15% carbon dioxide). When you crack your knuckles or tug on your fingers or toes, a low-pressure zone is created in the synovial fluid which draws CO2 and water vapour out of the solution creating an air bubble which collapses almost instantly, causing the popping sound.

Once the big gas bubble has popped, a little one remains behind for about 15 or 20 minutes before the CO2 inside it is totally redissolved. During that time, any further finger-tugging simply causes the micro-bubble to expand a bit, like a tiny shock absorber. That’s why you can’t crack the same knuckle twice in rapid succession.

There is a hypothesis that prolonged joint stress due to cracking knuckles may eventually lead to a higher risk of joint damage. This is not proven yet and if you continue your habit, the world might have the privilege to see the first victim of knuckle cracking damage and parents could continue to say ‘I told you not to do that.’

Movie magic: An unnaturally loud knuckle cracking before a Hindi movie fight scene shows off the hero’s (or villain’s) power and preparedness!