Why are some people double-jointed?

Hypermobility. Hyperlaxity. Hyperflexible. Hyperextensible. Double-jointed. These words all refer to a condition in which a person has extraordinary flexibility. Currently, there are several explanations for this flexibility that center around bone shape and body composition.

A joint is the area where two bones come together. People have the greatest flexibility around ball-and-socket joints, which involve the ball-shaped end of one bone fitting into a corresponding depression on the other bone. For example, the hip, arm, and shoulder are all ball-and-socket joints. The shallower the indention is, the greater the flexibility in movement. Thus, people who are double-jointed sometimes have shallower joints that allow a wider range of movement.

In other cases, double-jointedness is a result of especially soft cartilage or ligaments that are more elastic. Cartilage, which is composed largely of collagen, acts as a cushion between bones. Double-jointedness due to collagen problems may sometimes be indicative of a more severe condition, such as Ehlers-Danlos syndrome (EDS), which is due to a defect in collagen synthesis and manifests as extreme flexibility. As the connective tissue linking bones, ligaments also play a role in increased flexibility. Greater elasticity in either collagen or ligaments can allow bones a wider range of motion.

Along with increased flexibility comes a number of less desirable side effects. People who have shallow joints might find it easier to dislocate a joint. Those who have elastic collagen might be more easily bruised or develop joint pains with greater frequency. Thus, double-jointedness may be a gift or a curse, depending on how you twist the question.