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Cracking joints is the practice of manipulating one's joints such that they produce a sharp sound, likened to cracking (also likened to popping, etc.). The most common form of this occurs during knuckle cracking, a process different from snapping one's fingers. It is possible to crack other joints, such as vertebrae.^[1]

See also: Joint manipulation

To produce the clicking sounds, many people bend their fingers into unusual positions. These positions are usually ones that their own muscles are unable to achieve, and which are not commonly experienced in everyday use. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (distraction), compressing a finger knuckle toward the palm (into flexion), or twisting a finger about the first bone's axis (torsion).

Cracking within the body may also be caused by a breaking bone.

The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.^[2]

When a manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution creating a bubble or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. This process is known as cavitation. The contents of the resultant gas bubble are thought to be mainly carbon dioxide.^[3] The effects of this process will remain for a period of time known as the "refractory period", which can range from a few minutes to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.^[4]

The physical mechanism is as yet unproven, but suggested theories include:

1. Cavitation within the joint—small cavities of partial vacuum form in the fluid then rapidly collapse, producing a sharp sound. This explains the popping that can occur in any joint, such as during spinal manipulation. Synovial fluid cavitation is the most likely theory and substantial evidence exists in support of it.
2. Rapid stretching of ligaments.^[5]
3. Adhesions being broken, which simply means that as two cartilage surfaces are pressed together, they form adhesions, and when the joints are separated this makes the popping or cracking sound.

A single event is not enough to cause damage to the joint, although there is a hypothesis that prolonged joint stress due to cracking knuckles may eventually lead to a higher risk of joint damage. However, the long-term consequences of this practice have not been studied thoroughly, and the scientific evidence is inconclusive. The common parental advice "cracking your knuckles gives you arthritis" is not supported by any evidence, but habitual knuckle crackers are more likely to have hand swelling and lower grip strength.^[6]