Does external rotation exercise build any appreciable muscle mass?
I feel like trainers try to sneak them in on people like a mother trying to get her kid to eat peas. They are good for you as far as injury prevention, but aren't really going to build you muscle in a direct sense (though strengthening your over head pressing does).
However, after incorporating external rotation for a few months, my shoulders feel alot more solid, especially toward the rear. Am I building those tiny ERs or is it just improving my shoulder movemnets?