Thanks a lot for all your suggestions!
I'll take a look to the optical mouse idea, and to the gyroscope sensor too.
Just to make clearer what I need, here there is another image showing which movements I need to track:
More precisely I need to track abduction (rotation on X axis), flexion (rotation on Y axis), and external rotation (rotation on z axis).
About Jackrae suggestion
If you are measuring humerus relative to scapula (humerus-scapula) you will get away with only three rotation sensors (X, Y, Z).
can you advise some model of three rotation sensor?
Thanks again, Valerio