Muscular Discrimination

One thing I never claim to be is a biologist. So it is of little wonder that I'm confused about how the human body works.

Why is it that some muscles are meant to be worked and some muscles are meant to be left alone? I thought that muscles were muscles and when you worked them they got stronger. I mean, way back when people put their backs into it.
(pause for laughter)
And back then, these people were referred to as "strong backs." But it seems these days your back will only get worse and working your back will lead to "bad back." But none of your other muscles do that. You don't have people at the gym telling you not to lift that with your arms. Hell, people tell you to lift with your legs and then preach about how you're not supposed to strain your knees! How the hell do you lift with your legs and not your knees?

So if anyone is, or knows of, a biologist, please explain to me why my back isn't meant to be used, but all my other muscles need to go to the gym and work for no reason whatsoever!