Not too long ago back, I was pulling my hair out because I couldn't draw a male's back like I wanted to. Then, when I got that right, the bottom was giving me trouble. >.< Anywho, I was given a site from one of my needing help topics for such a thing. No, this site does not show naugthy parts. In fact, it just shows people in their muscle layer. It really help when you are trying to figure out how the body is suppose to look, when doing a certian type of pose. Here's the link:
www.posemaniacs.com/?cat=46