Why are some people double-jointed?

THAT'S THE WHY:Do you know anyone who can bend their thumb right back to touch the wrist behind it?

Or perhaps you are one of those lucky souls who can scratch behind their ear with their toes as a party trick.

Why are some people so bendy? Being double-jointed doesn’t actually mean you have two joints instead of one, but that the joint is hypermobile.

One factor that increases the range of mobility in a joint is the connective tissue, and genetic variations in the matrix proteins that make up these tissues can mean they are more supple in some people.

Another is the depth of the joint itself: having a shallow socket in a ball-and-socket joint may also extend the possibilities for movement.

However, sometimes double-jointedness can be a double-edged sword.

"For many it is an asset that confers greater facility for physical prowess. Others, less fortunate, fall prey to the associated effects of tissue fragility," states a paper in the International Journal of Clinical Practicein 2000. "The most frequently encountered constellation of traumatic and overuse injuries is termed the (benign joint) hypermobility syndrome (BJHS)."

Let’s hope that’s not an issue for Mongolian contortionist Iona Luvsandorj, who last year at age 28 set a world record by holding a “marinelli bend” for 33 seconds, supporting her bodyweight by gripping a stand with her mouth with her back arched over her head. Ouch.