Do you pop your knuckles or do your joints crack when you work out or even get up out of a chair? There's a simple scientific explanation for the phenomenon. To understand how the popping sound is produced, it's helpful to know how joints work.
How Joints Work
Joints are where two bones meet. The ends of the bones are protected from rubbing against each other by caps of cartilage. If the joints weren't protected, bone would grind against bone, which is painful as well as destructive. The articular cartilage is cushioned by viscous, clear synovial fluid, produced by a membrane that surrounds each joint. Synovial fluid lubricates joints, much like oil lubricates metal parts in your car engine, preventing hard parts from grinding themselves up.
What Makes Joints Pop and Crack?
When you pop your knuckles or crack any joint, you're pulling the bones in a joint away from each other. This opens up the space in the joint, reducing the pressure inside it. The lower pressure pulls gases dissolved in synovial fluid out of solution. When the oxygen and carbon dioxide become less soluble, they form bubbles. The pop you hear is the sound of bubbles forming, much like you hear bubbles form when you crack open a can of soda, lowering the pressure inside the can, so the dissolved carbon dioxide can form bubbles.
If you take an x-ray of a joint right after popping it, the bubble is visible. It increases the size of the joint about 15%. The bubble doesn't last forever, though. After about half an hour, the gases dissolve back into synovial fluid. Once you pop your knuckles, you can't pop them again right away, because you need dissolved gases to get the effect. Other pops and cracks in your joints that you can do over and over again most likely are ligaments snapping back into place.
Is Popping Your Knuckles Bad for You?
In either case, the popping sound may sound scary and annoy others, but there is no evidence that repeatedly cracking your joints is harmful. However, it may lead to weaker grip strength, possibly from stretching out the joint repeatedly.