Someone with a joint that abnormally bends in two directions, rather than one. For example, I'm double-jointed in my fingers, and can bend them backwards until my fingertips touch the back of my hand. I've known people who can bend their elbows backwards, and supposedly some people can do the same with their knees. The term is something of a misnomer, since we don't actually have an extra joint.

Back in middle school, being double-jointed was useful for freaking out girls and pretending that I'd gotten my hand slammed in a door. Nowadays, other than playing tricks on stoned friends, the best thing I can do with my talent is hold four soda cans in a single hand, and sometimes when people offer me a handshake, shake their right hand with my left.

An old-fashioned way to describe people with joint hypermobility, from the notion that their fingers and limbs could bend so far that there must be a second joint in them. Of course, they do not. Instead, their joints simply allow a greater range of motion than most; the five percent of the population with the greatest range of motion are medically considered hypermobile. Thus, double jointed-ness is not a syndrome, although hypermobility can be a symptom when brought on by joint instability or injury.

Generally, being double jointed just means that the cartilage between two bones is more supple than average, and will allow that joint more give. Children, especially girls, have much more flexible cartilage than adults and are thus more likely to be double jointed. If the joint isn't exercised and stretched as the child grows up, the cartilage may harden in place and lose the double joint. Shallow joint sockets are also a good predictor, having less bone mass around the joint means it can move more freely. Some peoples' joint sockets are so shallow that they can dislocate the joint at will, which is a striking thing to see but pretty hard on the joint's cartilage and ligaments.

In a study published in the Journal of Rheumatology, it was found that being double jointed correlates strongly with fibromyalgia, or at least some symptoms of it. Instead of being caused by hypermobility, the researchers hypothesize that the repeated minor trauma of moving joints out of their intended range may bring on local symptoms of fibromyalgia, which then diffuse into the surrounding areas. That is, being double jointed doesn't cause fibromyalgia, but may contribute substantially to its onset.

Having fingers that can collapse backward is something of a liability for instrumental musicians, as they depend on precise movements of all of their fingers. Woodwind players have particular trouble, and will probably be unable to reach their highest technical potential -- double jointed-ness is considered an irrevocable handicap. One web page suggests a somewhat creepy regimen for getting rid of one's finger hypermobility, by putting pressure on the joint while it is locked, but not collapsing it. This supposedly builds muscle strength in the locked position, and after a few months the finger should never collapse naturally again.

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