My buddies wife has a friend that teaches Yoga, but she does it from a scientific angle as opposed to a spiritual one.  I have never been, but I think I would enjoy that much more than the latter.  So for example, she will explain that a certain position positions a specific muscle at a right angle to another one thus allowing it to stretch more completely.  I made all of that up of course, but from what I understand that is the kind of stuff she explains to you as you are doing it.