Where does the term 'double-jointed' come from?

I've read many times recently that Michael Phelps is double-jointed. The term is totally misleading because it suggests that someone with this condition has twice as many joints as others have. Actually, the term double-jointed is connected to something called Hypermobility and means only that the joints can stretch farther than normal.

I wondered whether the word 'double' refers to the extent or degree of something in this context (not quantity), i.e. perhaps the joints can be stretched twice as much.

How come they call it double-jointed?

Solution 1:

Sadly, the OED (through the 1987 Supplement) does not provide an etymology. My guess is that it is a metaphorical extension from joinery or mechanics. There are many sorts of double hinges (often called 'double-action hinges'); or it would be very natural to refer to what we now call a universal joint or U-joint as a 'double joint'. All of these are designed to provide a rotatory joint with more freedom of movement than normal: either more degrees of rotation about a single axis, or rotation about more than one axis, or both.

Solution 2:

As I understand 'double-jointed' refers to the idea that such people have opposing joints, which allow them to "fold" their knees/elbows etc. in both directions with equal ease.

Naturally this isn't accurate as we know people considered to be double-jointed simply possess extremely flexible joints (or rather - ligaments) which allow them to stretch their appendages farther than most people.