Cracking knuckles is the practice of manipulating one's fingers such that the joints (most usually the knuckle joint) produces a sharp sound, likened to cracking (also likened to popping, etc). To produce the sounds, many people will bend their fingers 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, or pulling them away from the hand.

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:

1. 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)
2. the sudden stretching of ligaments (hypothesis by one reader of that journal)
3. release of gas from the joints being adjusted (this applies to the popping that can occur in any joint such as during chiropractic manipulation)

A single event is not enough to cause damage to the joint, although it is possible 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.

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." [1] (http://www.chiro-online.com/interadcom/murphy3.html)

Links

• Mechanisms Involved in Joint Manipulation (http://www.chiro-online.com/interadcom/murphy3.html) - from a Chiropractic website