You may have noticed pops and cracks when you move your joints, particularly after you've sat in place for too long. Your knees, ankles, the knuckles on your hands and your neck and lower back are particularly prone to these popping sounds. Although the exact mechanism behind these noises is not known, there are a number of explanations strongly favored by the scientific and medical communities.
One of the most popular explanations is that the synovial fluids in your popping joints cavitates — as external forces are put on the liquid as you move, tiny bubbles form in the fluid and implode. This is what causes the popping and cracking sounds. Cavitation is one of the most popular theories because a number of scientific studies back it up. Unfortunately, the pops could have health implications because cavitation can cause damage in structures other than the human body. For example, cavitation is infamous for being able to pit and even eat away at the metal in pumps and propellers over time — as the collapsing liquid is forced into a small space, it produces high temperatures and damaging shock waves.
Two other possible causes of popping and cracking noises when you move your joints are that either your tendons are shifting in and out of place as your move, or that your ligaments were being stretched too fast. Neither scenario is a health concern barring unusual and extreme cases.
A third scenario for popping and cracking noises is the movement of the joint rubbing against rough surfaces. Specifically, the movement of the joint against rough surfaces indicates that the cartilage has been worn away and the two bones in the joint are rubbing against each other, a sign of arthritis, particularly when accompanied with pain.
In general, though it has been shown that regularly popping joints does not increase your chance of arthritis, the science is still unclear on whether popping joints are bad for you.