The causes of the noise when people crack or pop a joint include bubbles from escaping gases, rough joint surfaces and joint movement, according to the Library of Congress. Joint capsules protect joint bones and the cartilage that covers them. Synovial membranes line the capsules and produce a protective lubricating fluid containing carbon dioxide, nitrogen and oxygen gases. When people crack or pop joints, they are stretching the capsules, which release the gases, resulting in bubbles that cause the noise.
When joints lose smooth cartilage, as occurs in people with osteoarthritis, the joints make sounds as they move because of the roughness, explains the Library of Congress. As people move their joints, the corresponding tendons change their positions, moving out of place. When a tendon returns to its original position, it often produces a snapping sound. People moving their joints may also feel their ligaments tightening, especially in the ankles and knees. The tightening often creates a cracking sound.
People who experience pain when their joints pop should consult physicians, notes the Library of Congress. While some study results show that knuckle cracking is not harmful, other studies show that soft tissue damage can result from continual knuckle cracking. Swollen hands and a weak grip can also result from the habit.