Cracking joints

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Cracking joints is the action of moving joints to produce a sharp cracking or popping sound. The most common form of this occurs during deliberate knuckle-cracking. It is possible to crack many other joints, such as those between the back and neck vertebrae, hips, wrists, elbows, shoulders, toes, ankles, knees, jaws, and the Achilles tendon area.



To deliberately 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. However, cracking a joint that has been exercised recently is generally recognized to be palliative. 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 (torsion).

The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.[1]


The physical mechanism is uncertain. Suggested causes include:

  • Cavitation within the joint—small cavities of partial vacuum form in the fluid and 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. Cracking knuckles releases gases from the joints.[2]
  • Rapid stretching of ligaments.[1]
  • Intra-articular (within-joint) adhesions being broken.[1]

Of these hypotheses, perhaps the most popular is cavitation. 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, making 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 seconds 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]

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