The act of cracking joints means bending a person's joints to produce a distinct cracking or popping sound, often followed by a feeling of satisfaction or relaxation to the person. It is sometimes done as part of a joint adjustment routine performed by a chiropractor.
According to a folk belief, the popping of joints, especially knuckles, leads to arthritis or other joint problems. However, medical research has so far failed to demonstrate any connection between knuckle cracking and long-term joint problems. While the cracking mechanism and the resulting sound is not yet fully understood, it is mainly attributed to nitrogen or carbon dioxide bubbles building up between the joints.
The physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints is uncertain. Suggested causes include:
There were several theories to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it. When a spinal 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, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of the resultant gas bubble are thought to be mainly carbon dioxide. The effects of this process will remain for a period of time known as the "refractory period," during which the joint cannot be "re-cracked," which lasts about twenty minutes, while the gases are slowly reabsorbed into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.
However, recent evidence demonstrates that the cracking sound is produced when the bubble within the joint is formed, not when it collapses.
The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.
The common claim that cracking one's knuckles causes arthritis appears unsupported. A study published in 2011 examined the hand radiographs of 215 people (aged 50 to 89) and compared the joints of those who regularly cracked their knuckles to those who did not. The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles. An earlier study also concluded that there was no increased preponderance of arthritis of the hand of chronic knuckle-crackers; however, habitual knuckle-crackers were more likely to have hand swelling and lowered grip strength. Habitual knuckle-cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol and was suggested to result in functional hand impairment. This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.
Medical doctor Donald Unger cracked the knuckles of his left hand every day for more than sixty years, but did not crack the knuckles of his right hand. No arthritis or other ailments formed in either hand, earning him the 2009 Ig Nobel Prize in Medicine, a parody of the Nobel Prize.