When someone displays a unique and extraordinary dexterity and can extend his arms beyond the limits that most people can, he is labelled as double-jointed. But what does it mean to have a double-jointed arm -- how does it happen, and what allows some people to have the ability and others not? Understanding double-jointed arms is a matter of examining the physiology that constitutes hypermobility.
Generically, people call anyone with a unique ability to bend her arms farther back than anyone else double-jointed. The medical term is hypermobility. At times, it can be referred to as hypermobility syndrome, benign joint hypermobility syndrome or the more technical-sounding hyperlaxity. Although it is most noticeable in the elbow joints, hypermobility is not limited to the elbows, and can apply to the shoulder blades, fingers, ankles and even legs. It denotes the ability to extend the ligatures around the joint farther than ordinary.
When the ligature around the elbow joint or the arm's socket joint is able to flex farther than normal, an arm will appear to be distended. Ligaments function much like ropes that hold a boat steadfast or are used to pull it to shore: as long the ligature muscle has slack, a "double joint" will continue to move until the ligatures become taught. Sometimes this indicates a problem. Usually though, it is completely benign.
Hypermobility can denote problems, most notably a misaligned joint; abnormally shaped ends of bones and joints; or a type of connective tissue disorder such as Marfan syndrome, which indicates an ongoing problem with the bones. If you aren't sure whether hypermobility is a problem, then show it to the doctor during your next checkup. If a recent accident proved difficult to recover from or you've suffered a break that affected your joints, then monitor yourself for newfound hypermobility, and point it out to your general practitioner.
Living with Hypermobility
Having hypermobility in your arms comes with advantages -- most notably the novelty of showing friends and family. If you have double-jointed arms, then be cautious about routinely overflexing, as the wear on the joint bones can lead to osteoarthritis. Studies have shown a link between hypermobility and anxiety, and that the condition can be indicative of both fibromyalgia and chronic fatigue syndrome. Again, monitor yourself for changes in your joints.