I had wanted to make this for years now. This is a solution I have found to be able to pose the shoulders of my human models to a wide range of positions, looking good and without distortions.
There is a lot of work involved, and it is intended for advanced modelers, since I don’t approach the needed modeling with much detail, although anyone with enough curiousity might find it interesting.
If you are having basic problems with the shoulders of your models, like weight painting, this is not for you.
Although it actually works, my understanding of the involved math is very limited. If you can think of a better way to get the X and Z rotation of the upper arm, disregarding any Y rotations, that would be a welcome improvement.
Here is the link: