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Cracking or popping of joints is the action of joint manipulation to produce a sharp cracking or popping sound. This commonly occurs during deliberate knuckle-cracking. It is possible to crack many joints, such as those in the back and neck vertebrae, hips, wrists, elbows, shoulders, toes, ankles, knees, jaws, feet, sternum, and the Achilles tendon area. Although it is a common folk belief that popping of joints, especially knuckles, leads to arthritis or other joint problems, medical research has so far failed to conclusively demonstrate any connection between knuckle cracking and long-term joint problems. ==Causes== The physical mechanism causing a cracking sound produced by bending, twisting, or compressing joints is uncertain. Suggested causes include: * Cracking sounds occur when bubbles form in joints as they are pulled apart * Cavitation within the joint—small cavities of partial vacuum form in the synovial fluid and then rapidly collapse, producing a sharp sound. * Rapid stretching of ligaments.〔 * Intra-articular (within-joint) adhesions being broken.〔 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 back 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cracking joints」の詳細全文を読む スポンサード リンク
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