Joint cracking occurs when gas bubbles in the joint pop. However, it could also occur by the movement of tendons and ligaments rubbing over bony parts of a joint.
In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed. In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles’ collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion.
The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound. For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain. Suggested causes included:
Studies show that as many as 54 percent of people crack their knuckles. They do it for a lot of reasons, including: