Cracking knuckles

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.

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)

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.

External reference

Here is a discussion of existing medical papers on the topic of cracking knuckles with commentary.

http://www.urbanlegends.com/medical/knuckle_cracking