Why Do Knuckles Crack?
Surprisingly, scientists aren’t sure about this one, but they have two ideas
You’d think scientists would have this one figured out. They sort of have. The pop we hear when we crack knuckles or other joints involves gas bubbles within the fluid that lubricates joints. But exactly how that air makes noise is a bit, well, up in the air.
What we do know is that the cracking happens when the space between joints is increased, by bending or pulling. This creates a pressure vacuum within the synovial fluid, as it’s called, and dissolved gases form an air bubble. What we don’t know is whether the sound is caused by the bubbles forming or collapsing.
It’s hard to get inside a knuckle and watch what’s happening, but researchers did the best they could in a 2015 study, using an MRI machine. They concluded the pop is made by air rushing into a cavity, forming a bubble. Another group of scientists, using a mathematical model in 2017, argued that the collapse of a bubble would produce a sound consistent with what we actually hear.
Either way, the air bubbles help explain why we often can’t crack the same joint twice without a delay: The air has to build back up (or dissolve back out, depending on which explanation is correct). There’s also room in all this for a partial crack, scientists say, which sometimes allows for a second cracking right away.
So, why do we do it? For some it’s a nervous habit, a way to release mental energy. Kids probably find the sound pretty interesting. My experience, glued to a keyboard for hours and hours every day, is that my cramped hands just need a release now and then, and experts agree joints feel looser after a good cracking.
Importantly, and contrary to what Mom said, knuckle cracking does not seem to cause arthritis. A large study in 1975 found no greater risk for arthritis in older people who were lifelong, habitual knuckle-crackers. The finding was confirmed by a smaller 2011 study.
But there’s at least one reason to kick the habit: It may reduce grip strength. Plus, it annoys the hell out of some people.