Science explains: cracking down on knuckle cracking

You either relish the sound as it hits you or cringe in disgust when someone feels the need to do it – whatever your reaction, knuckle cracking tends to provoke one. And scientists love it too – from old wives’ tales about knuckle cracking giving you arthritis to theories about your knuckles filling with air, they’ve been eager to analyse the phenomenon and explain where it comes from. How do our knuckles crack, and why?

A number of conflicting theories about knuckles have been proposed in the past. In between the knuckles, there’s a fluid – the original thought was that bubbles (made up of carbon dioxide and other gases) formed when the space between the knuckles increased. There were two rival ideas here – that the cracking sound came as a result of the bubble collapsing (when you cracked your knuckles) or the pressure change as the bubbles formed (you change the space when you crack your knuckles).

In between the knuckles, there’s a fluid – the original thought was that bubbles formed when the space between the knuckles increased

A 2015 study appeared to provide the solution – a number of researchers in Canada saw knuckle cracking take place in an MRI scanner, and they came to the conclusion that the cracking sound was down to the rapid separation of the joint and bubble formation, rather than any bubble collapse.

That’s the issue put to bed, right? Not exactly – Professor Abdul Barakat of the Hydrodynamics Laboratory at École Polytechnique noted that the imagining techniques used in 2015 weren’t sufficient to capture the high-speed dynamics of knuckle cracking. As a result, Professor Barakat teamed up with one of his students to develop a mathematical model to explain what was going on. This model was based on three components – the change in pressure of the fluid as the knuckles move apart, the growth and collapse of the resulting bubble, and how changes in pressure from the bubble turn into sounds.

That’s the issue put to bed, right? Not exactly

They compared their model to sound patterns of knuckle crackers, and they found a positive correlation (they also debunked the theory that the formation of bubbles explains cracking, with the formation failing to produce sounds of the observed loudness). This model helped explain the 2015 study, suggesting that more did indeed go on than could be observed, and the fact that bubbles remain in the joint fluid – a partial collapse of the bubbles is all that’s needed for the sound, and tiny bubbles are left over as a result. It also confirms why some people can’t crack their knuckles – if there’s a large enough space between the bones in the knuckles, the pressure doesn’t drop low enough to trigger a sound.

So, that’s how your knuckles crack – why do some of us feel the need to crack? Essentially, it’s down to how much you use your hands and fingers throughout the day, and whether you do any stretching otherwise – for people like writers, coders or surgeons, whose fingers are effectively always in use, you’ll feel the need more often. But it goes further than that, once you get into the habit of cracking your knuckles, it essentially becomes an addiction. No matter how much other people complain, it may be the case that you simply cannot stop (if you’re particularly worried about knuckle cracking addiction, perform regular hand, wrist and forearm stretches, and that should relieve the need).

Once you get into the habit of cracking your knuckles, it essentially becomes an addiction

At the moment, these results are simply a potential explanation, and further experimentation needs to be done to validate whether the model is actually able to finally explain what’s going on in your knuckles. While we wait for these experiments to go on, here’s a tip from Professor Barakat if you are able to crack your knuckles and you enjoy it: “The more rapidly you pull on your knuckle, the faster you are changing the pressure and therefore the more likely you are to generate a knuckle crack.” Fast and smooth, and you’ll get a good solid crack – just make sure you’re doing it in appreciative company!