What IS double-jointedness?
No one has extra joints in there body! People who are considered "double jointed" simply have more flexibility in their joints than most people. Why are they so much more flexible than others? It can depend on many things such as your age (younger people are more flexible), gender (women are more flexible) or bone structure (flatter surfaces allow for more movement).
Physiotherapists and massage therapists have a scale that can measure exactly how flexible you are compared to other people. The Beighton Scale consists of 5 different movements and is scored out of 9. If adults score higher than 4/9 they are considered extremely hyper mobile.
It is very important to balance your flexibility with strength. If you are too flexible and your body does not have the strength to support it, you open yourself up to preventable injuries. If you feel that you are overly flexible or not flexible enough, it is a good idea to book in with a physiotherapist or massage therapist to determine the best way to protect your joints.