Everyone has heard of someone who claims to be double-jointed and is able to pull their thumb to their forearm. While this is not considered that much out of the ordinary, there is no such thing as being double-jointed. Two bones meet at a joint and their motion is limited by two the ligaments and the contours of the bones and cartilage.

While most of people can bend at the waist, some are able to bend much more and are able to place their hands flat on the floor. Medically, this is called hyper-mobility, either because one is able to super-extend their ligaments or  because of the placement of their bones. In order to classify as being hyper-mobile one must possess this naturally, not due to training.

One example is that of the elbow and the ability to swing the forearm beyond 180 degrees. Some people are able to bend this joint in the wrong direction. The tip of the elbow, has a big hook that fits into a small groove at the back of the long bone in the arm. When the arm is extended, the hook slides into the little groove such that when it hits the end of the groove, the elbow cannot extend further. This means that if the tip is small, the elbow is able to extend more than that of other people as the bone is stopping it further away, or the ligament is more flexible.

While being super-flexible may seem like a plus for dancers, this could prove to be more strenuous as dancing also requires extreme control and precision. Ultimately, this misconception is due to the choice of language, as no one possesses extra joints.