Comment & Analysis
“THE CRACK COMES FROM THE SUDDEN CREATION OF THE CAVITY, AND IT’S LOUD BECAUSE IT’S A VIOLENT PROCESS”
Helen Czerski on cracking knuckles
Along with petting puppies and having just one more chocolate, I’ve added another thing to my list of temptations that humans can’t resist: knuckle-cracking. I mentioned this odd habit in a talk that I used to give quite regularly, and almost as soon as the words were out of my mouth, the popping from the audience began.
In the most extreme cases, usually large auditoriums full of teenagers, it would grow like an avalanche: a few lone snaps at the back of the room rolling forwards in an impressive crescendo as everyone else joined in. But finger joints are tiny, and the process of cracking (pulling on a finger joint until it pops) is relatively gentle. What’s making this distinctive sound and why is it so loud?
The first step is a bit of anatomy. A joint is where two bones meet. The ends of your fingers’ bones are covered with a layer of cartilage, but between the two cartilage layers is a cavity filled with gloopy stuff called synovial fluid, which lubricates the joint as the bones move. Having that fluid-filled cavity there means that the joint can lengthen a little bit – there’s no rigid structure holding the whole thing together. The would-be knuckle-cracker pulls on one of the bones, applying a force that could stretch the joint. The interesting thing is that nothing happens immediately. To pull the bones further apart, you need something that’s going to fill the new space you create. And then… pop! There it is: a bubble.
This does sound a bit like magic, but the gas was there all along, dissolved in the synovial fluid. Liquids don’t stretch much, so when you put a liquid under tension, the only way for it to expand significantly is for it to pull apart and create a hole. Then gas that was dissolved in the liquid will escape because the pressure has dropped (a bit like gas coming out of solution in a fizzy drink when you take the lid off). The crack comes from the sudden creation of the cavity, and it’s loud because it’s a violent process.
The bubble will sit there until the gas redissolves, which is why the musical contribution of my audiences was short-lived. Once you’ve already got a bubble, gas can move from the liquid to the bubble quite easily, and if you pull on the joint, the bubble will just expand slowly to fill the gap. It takes about 20 minutes for the bubble to dissolve, and you won’t get another crack until you’re able to form another bubble from scratch.
But the reason I mentioned knuckle-cracking in my talk at all was to get to the tale of Donald Unger, who won an Ig Nobel Prize in 2009 for his long-term and very personal investigation of the topic. Having been told as a child that this ‘bad habit’ of cracking his knuckles would lead to arthritis, he cracked the knuckles on his left hand (but not his right) twice a day for 50 years – just to see what happened. At the end of that time, he didn’t have arthritis in either hand, and subsequent medical science has backed up the idea that it won’t do you any harm.