When a woman stretches her arm back fully her forearm goes back beyond straight; the arm goes back beyond 180 degrees.Now no man I know can do this on asking. So is this bad observation or true anatomy? I am sure I read somewhere about the female hips and arms being adapted for each other and for lugging infants about but that may have been some male's excuse for not doing it !

Oh no I am defective! Mine does not go past 180.

But you made me look.

I'm not sure I understand the position you are referring to. Here is a picture of a woman stretching her arm back at 45 degrees doing what is called an Extension. (scroll down to Movements of the Shoulder - 2nd picture)

The functional range of arm motion is 70-110 degrees with 180 degrees being the maximum for extremely flexible people. At 160 degrees your rotator cuff (the 4 muscles that reinforce the head of the shoulder joint during movement) becomes inactive and any motion past that degree can cause tearing.

I'm referring to the elbow joint itself. When the women around here stretch up their arms straight and rigid, there is a marked backward kink, slightly beyond what seems to be straight. None of the men around here do this. It is not a conscious action. It can be seen when she jumps up and cheers, for example throwing her arms up straight or even stretching the arm up and waving by wiggling the hands.

Guess you will have to draw me a picture. When I raise my arm straight over my head with the palm facing forward my elbow indentation is lined up with the side of my torso.

Most references I've seen give the normal range of motion (ROM) of the elbow as 150 degrees full flexion (forearm maximally bent) to 0 degrees full extension (forearm fully straightened), but I recall at least one reference that allows the normal ROM to extend to -10 degrees extension, i.e., 10 degrees hyperextension. I am not aware of gender differences here.

It is well known that the carrying angle of the elbow is slightly greater in women than men. This refers to the fact that, standing with the arms dangling at your sides & palms facing forward, the forearm deviates slightly laterally (cubitus valgus) from the upper arm, lest your hands smack your hips when they swing during walking!

In men 5-10 degrees carrying angle is normal, women 10-15 degrees. Whether this is related to the greater width of the female pelvis, and whether this can be derived by Darwinian versus teleologic reasoning, is not territory into which I wish to venture...