Quirky Body Series

Why Do We Crack Our Knuckles?

For knuckle-crackers, the satisfying "popping" noise may be music to their ears. For those around them, it can be creepy and annoying. Whichever camp you fall into, you may have wondered what causes the sound and whether it's harmful.

The "pop" of a cracked knuckle is caused by bubbles bursting in the fluid that lubricates your tendons and joints, called synovial fluid.

When you pull the bones apart, either by stretching the fingers or bending them backward, you stretch the capsule containing the synovial fluid. This decreases the pressure in the capsule, and decreased joint pressure stretches the gas-filled bubbles. If stretched enough, they burst.

Ever wonder why you can't crack the same knuckle twice in a row? It's because you have to wait for gas to once again build up in the synovial fluid.

A lot of people think that knuckle cracking can lead to arthritis. It may seem like a logical connection, but it hasn't been supported by research. For example, one study that compared rates of arthritis among 74 habitual knuckle-crackers and 226 people who didn't crack their knuckles found no difference in the rates of arthritis between the two groups.

Still, the authors of that study didn't seem to endorse the habit when they compared the sudden, vibratory energy produced during knuckle cracking to "the forces responsible for the destruction of hydraulic blades and ship propellers." And maybe with good reason chronic knuckle-crackers were more likely to have swollen hands and reduced grip strength.

There are at least two published reports of injuries suffered while people were trying to crack their knuckles. Perhaps these people were overly vigorous in their efforts. Fortunately, such reports are quite rare.

While knuckle cracking can be annoying to others, it seems to be harmless. The same is generally true of other joint-related noises such as popping, crackling, or snapping, as long as no pain comes along with it.

Here are some reasons to be concerned when your joints are noisy:

  • If pain accompanies a grinding sound during joint movement, it could be a sign of osteoarthritis or another form of joint disease.
  • If the joint seems unstable, for example if it locks or gives way, a "clunking" sound could be a sign of torn cartilage, dislocation, or another internal joint injury.
  • If you hear a sudden "pop" and experience pain while you're exerting a nearby muscle, you may have just torn a tendon.

So, if pain accompanies the noises your joints make, see your doctor.

If you're a habitual knuckle cracker, rest assured that you're probably causing no harm. And if you're trying to get someone to stop cracking their knuckles, tell them it bothers you. Perhaps that will be enough. But, don't tell them it's bad for them or you'll just be spreading another medical myth.

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