Cracking joints

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See also: Joint manipulation

Cracking joints is the practice of manipulating one's joints such that it produces a sharp sound, likened to cracking (also likened to popping, etc.). To produce the sounds, many people will bend their fingers or other joints into unusual positions, or manipulate them in ways which are not commonly experienced in everyday use. For example, bending the fingers right back towards the back of the hand, pulling them away from the hand, or compressing a finger knuckle palmarly.

In many early motion pictures and subsequently parodied in animated cartoons, the gesture of cracking knuckles was associated with a "tough guy" image, especially when accompanied by the implicit or explicit threat of violence.

The physical mechanism is unknown, but possibilities that have been suggested include:

cavitation within the joint – small cavities of partial vacuum form in the fluid then rapidly collapse, producing a sharp sound (hypothesis in a medical journal)
the sudden stretching of ligaments
release of gas from the joints being adjusted (this applies to the popping that can occur in any joint such as during chiropractic manipulation)
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.

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

A single event is not enough to cause damage to the joint, although there is a theory that prolonged joint stress due to cracking knuckles may eventually lead to a higher risk of joint damage. The long-term consequences of this practice have not been studied thoroughly, and the scientific evidence is inconclusive. However, the common parental advice "Cracking your knuckles gives you arthritis" may be simply a device to deter children from this practice, because many people find the sound annoying. In fact, a study by Castellanos and Axelrod found that arthritis was not a product of knuckle cracking. They evaluated 300^[1] consecutive outpatients at Mount Carmel Mercy Hospital to determine whether habitual knuckle cracking is a risk factor for hand dysfunction. They found no relation with osteoarthritis, but noted that 'knuckle crackers were more likely to have hand swelling and lower grip strength.'

A chiropractic perspective:

As a joint is being distracted, the capsule invaginates inward and as the stress on the capsule reaches a certain threshold, it suddenly snaps back from the synovial fluid, increasing the volume of the capsule (and decreasing pressure) and causing the audible sound. The sudden increase in the volume causes the tension to drop, allowing the joint to increase in movement. Eventually, the elastic limit of the capsule is reached and the process stops. The time elapsed during all this is shorter than that required for completion of the stretch reflex, so it can occur without muscular resistance. The sudden jerk on the capsule and the other periarticular tissues is theorized to cause firing of the high-threshold mechanoreceptors^[2].

Another alternative description can be found in the e-book The Missing Owners Manual which describes the process of adhesions forming between cartilage surfaces^{[citation needed]}, and how chiropractic releases those adhesions^[3].