Answer: Gas Bubbles

Although the act of putting your joints, most commonly your knuckles, under tension until you hear and feel a satisfying crack is referred to as “cracking” the joint. That’s a bit of a misnomer and we should really be saying “pop your knuckles” instead.

Why pop and not crack? Although the sound might have a cracking quality to it, you’re not really cracking the bones or even joints. You’re actually popping tiny bubbles of compressed gas. In between your joints you’ll find cartilage and around the cartilage you’ll find a substance known as “synovial fluid” that helps lubricate, protect, and nourish your cartilage the way oil protects an engine. The fluid is filled with cells that deliver nutrients to your cartilage and during the whole process of protecting and restoring the cartilage gases are produced as by products and gather in a suspended solution within the fluid.

At a certain point the gases reach a certain degree of concentration and pressure so that if you bend the joint just right, like when you flex your fingers to crack them, the gases will rapidly come out of the synovial fluid solution they are suspended in and crack!, you hear the familiar sound of your knuckles cracking. That also explains why people feel a sense of relief when they crack their knuckles and other joints. The build up of gases can be felt and the release of the pressure created by the concentrated gases is pleasurable and, thankfully, harmless. Despite what your grandmother might have told you (and scolded you over), there is no evidence that cracking your knuckles, neck, ankles, or any other common join that snaps, crackles, or pops, will cause arthritis or any other long term ailments (although researchers have found that cracking a joint over and over again can lead to temporary localized swelling).