Thanks Alexander!
I’ve been wondering about this for years, but never actually got deeper into it.
Today was one of those days that the idea would not let me alone and it was actually the image on the link above ( http://biology.clc.uc.edu/fankhauser/Labs/Anatomy_&_Physiology/A&P201/Articulations/supination_pronation_labeled.jpg )
that did it for me : when you look at the two images you can clearly see a gap between the capitulum and the ulna when in pronation, and
not so in supination, and the “hollow” form of the ulna actually allows for the kind of rotation that I’m describing, so that is the only explanation
I’ve been able to find that explains why the ulna appears to be rotating like the radius.
Would love to talk about that with an expert, because indeed anatomy books do not cover this in deep, and two-dimensional images only can learn you so much.
I’ve seen Youtube videos on the subject matter but in those they use 3D models, which is also not very accurate.
I’ve been reading on websites for radiologists and colleges and such but have not really found what I’m after.
What I’ve initially learned years ago is that the ulna does not rotate at all : I know this has to be wrong from experience : just hold your elbow, rotate your forearm in a way
that the hand keeps “occupying the same space” : you’ll see that the ulna has to rotate somehow otherwise your hand would not be able “to occupy the same space” : your
hand would be like rotating around the axis of the ulna if you know what I mean.
And I’m not talking about the rotation of the humerus : I’ve tested it with my elbow pressed against the table so I would not be able to rotate my upper arm.
So seeing that above mentioned gap between the ulna and the capitulum and looking at the form of the ulna at the capitulum I was thinking that my description is the only
possible way that the ulna can rotate, since it is impossible to rotate the ulna along its length axis (without breaking it), it has the able to rotate perpendicular to that axis, otherwise
your hand would not be able “to occupy the same space” when twisting your forearm.
Any further info is of course most welcome.