Knuckle Crackers

When I was a kid, few things were more satisfying to me than cracking my knuckles.

I was warned that this would lead to arthritis, but even knowing that, I accepted the trade-off and did it anyway. It’s a good thing this one turns out to be one of those myths we grow up with, but which aren’t really true.

What’s going on in there, though? We have some idea, but we don’t know everything.

The most widely accepted explanation is called cavitation theory. Fortunately, it’s pretty easy to imagine what’s happening. If you’ve ever seen someone spray a can of WD-40 at something to lubricate a hinge, you already have the idea.

Your joints are hinges. The WD-40 is called synovial fluid.

When you stretch a joint, it creates negative pressure inside the capsule—the little pocket that encapsulates the joint inside your body. This pressure causes gas bubbles to form from the gases that are dissolved in the fluid.

Gas bubbles can enter the fluid from the surrounding tissues, and when the pressure gets low enough, nitrogen (and a little oxygen and carbon dioxide) from your body gets in there, creating little bubbles.

You could imagine that your joint is now like fizzy water, if you like. After some time, you have to burp (or fart) to get rid of the excess gas you swallow along with the carbonated beverage. Instead of burping, you burst these tiny bubbles when you crack your joints.

Only, that’s not entirely accurate—I’ve misled you a bit here. Many researchers actually think that what causes the sound isn’t the bursting of the bubbles, but instead the formation. That’s pretty nonintuitive to us burping farters, but there’s good reason to think it might be the case: MRI results show that bubbles persist, even after a really satisfying crack.

I’m certainly no researcher with an MRI, but I do think I understand more than the average bear about joint function. Ever since 1997, I’ve been practicing martial arts where isolating and locking out a limb is encouraged.

Locking out an elbow is really different from cracking a knuckle. For one thing, it’s a one-way trip—you can’t break your arm because you’re bored, then come back in 20 minutes and break it again, the way you can return to crack a joint again.

The damage is typically to the connecting ligaments, and the threat of an armbar is to snap one of these important hinge pins at the elbow joint. Nevertheless, there are sometimes some very awkward moments when you’re rolling and you catch someone in a joint lock, only to have their "knuckle” pop.

This can be really scary, especially since you’re likely very attuned to what your partner’s body is doing in that moment, and the last thing you want to do is injure them. No, you want them to tap out here, to submit. My stomach always drops through the floor for a split-second any time a knuckle cracks when I have a submission in place.

All of our joints have synovial fluid in them. They all operate on similar principles, and that means they can all be cracked in a way. I’ve managed to crack everything from my elbow to my jaw at various times, and jiu jitsu has helped me to explore the knees, ankles, hips, elbows, shoulders, and wrists of thousands of grapplers.

What about you—did you crack your knuckles when you were growing up? Was it just me, or did everyone tell you to stop doing it, or else? Do you still crack knuckles today?