The joints are the meeting points of every bone, which holds together the connective tissues and ligaments. Synovial fluid, a thick and clear liquid, covers the joints in the body. When stretching or bending the fingers to pop the knuckle, you are making the joints pull apart. The connective tissues are stretched, increasing its volume and decreasing its pressure. Gases are dissolved in the fluid, as the pressure of the synovial fluid drops. It forms the bubbles through a process called cavitation. After stretching, the pressure in the capsule drop so low that it causes the bubbles to burst. It causes a pop sound that many people associate as knuckle cracking.
According to Anatomy and Physiology Instructors’ Cooperative, a study was done to prove if it has links to arthritis. It shows that out of 300 knuckle crackers, there were no connection between their habit and the ailment. Click here for more information about this study.