Double Jointedness
 

It has recently come to my attention that most people's thumbs don't bend backwards.

I always assumed that people's "last joint" on the thumb was supposed to allow about as much mobility bending forwards as it does backwards.

Now, I can move my thumb back and forth (that joint has 180 degrees of range) using my muscles. For my other fingers, I have to bend the finger with my other hand. But I end up with about 135 degrees before my joints actually protest.

How rare is this kind of double jointedness?

None of my fingers can bend backward. But I am double jointed in my elbows.

I knew a girl whose thumb could do the exact thing you described. She was a waitress, and would always freak people out when she served their dishes.

I've only got the thumbs you mentioned. Love pressing elevator buttons with an audience. No fingers though.

With help from the opposite hand, my daughter's thumbs can be bent to touch the wrist. We found this out when she was 3 and I took her to an ortho because of her turned-down flat feet. Doctor took her thumb, bent it down and declared she had 'hyperextensive' joints, meaning the cartilidge was extra pliable and the joints loose. She's now 14, can still do the thumb thing and, as a dance student, can do some really painful looking splits. And her feet are still weird.

Definitely not this flexible. I do have a thumb that pops out of joint and back in, so that one has a great range of movement. Still not like your description.