Why Do Knuckles Crack?

Apr 11, 2018 By Caitlin L, Writer Intern
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We’ve all heard the sound of cracking knuckles. For some, it is a guilty pleasure. For others, the sound is simply irritating. But what causes this sound? 

Scientists agree that the popping sound in knuckle cracking has something to do with the bubble formed due to the rapid release of pressure on the joints. However, there are two contradicting theories. An earlier study revealed that the crack comes from the bubble collapsing. More recently, scientists hypothesized that it could be due to bubble formation, but the MRI evidence demonstrated that bubbles remained after the popping sound.

To determine which is theory is correct, a team of researchers created a mathematical model of knuckle cracking.

The Anatomy Of Our Hands

The reason for our hand’s dexterity lies in its structure. The hand is made of 27 bones. Our palm is made of five long bones, called metacarpals. Each of our fingers has three small bones (except the thumb, which has two) known as phalanges. The places where these bones meet are called joints.

The knuckle is a joint and while usually, knuckles refer to joints in the hand, they can also be used to describe joints elsewhere. To prevent the bones from scraping against each other and allow joints to bend smoothly, synovial fluid fills the gap between them, acting as a lubricant and absorbs shocks. These joints are supported by ligaments to enable movement.

When you pull on your fingers to get your knuckles to crack, it causes the space between two bones to increase and simultaneously decreases the pressure of the synovial fluid. As a result, the synovial fluid expands and fills with gas pockets, which eventually collapse. Not everyone is able to crack their knuckles because, for some, the gap between the bones is too wide to produce a gas bubble.

Popping Gas Bubbles

Researchers Suja and Barakat saw a report from 2015 that showed a knuckle crack in each finger. From there, they created a mathematical model to find out where the sound came from. 

The three parts of the model deal with the pressure of the fluid, the relationship between bubble size and pressure, and sound emitted based on size and pressure. They played the model simultaneously with recordings of actual knuckle cracks, and the sounds proved to be in sync and consistent. They concluded that while collapsing bubbles is what causes the sound, the entire bubble does not need to collapse. It only needed to reduce by 30 to 40 percent to produce the sound.

A common misconception of cracking knuckles is that the action can cause arthritis or damage one’s joints. However, in reality, no damage is being done to one’s joints - it is just an air bubble forming!

Sources: Guardian, NYTimes, Nature, BBC, Arstechnica