Well, you did a good enough job in the pic with both Vault and Franz in it. :P

Anyway, your own body is the best tutorial, really. Whether or not you have any muscle tone, you can still feel over your own body to judge how the muscles stretch and bulge along with certain movements. It's also a pretty good idea to just draw from references, and eventually you'll get used to seeing bodies enought hat you can have a general idea how to alter a picture and keep the muscle tone realistic.

http://www.gaybodyblog.com/category/erotic-body - This blog here is INCREDIBLE for finding good refs to draw from, so check that out.

Pffffffffff. It's only decent because it's so sketchy that I don't have to have exact details. Ref pictures are jussst the kind of thing I need though. You win an internet.