When I started to get "serious" about anatomy I used the Burne Hogarth's dynamic anatomy book, that helps ya with all the muscles, mucles names, and even some forshortening.
That would lead ya somewhere, but it's not nearly enough...
As peeps say, life drawing, that really helps you, you need to see how the body changes when it's in motion. When a person stands still in real life, they don't stay rigid like those anatomy/proportion drawings that you tend to see everywhere. Even when your figure it's standing still (sitting, crouching, etc) the viewer should see that the body is capable of motion. Is not a rigid plastic doll...
Life drawing helps you with that, so if you can't go out there (or u don't like to), there are pretty good sites with naked models doing pretty cool poses that convey motion (even if they are sitting).
Also anatomy and proportions are good things to study together, you can know how to draw a perfectly anatomically correct arm, but if you always draw it 2 times larger than what it should, it may look off with the rest of the body (unless that's what you're looking for), so is good to take that into consideration when practicing anatomy, to see how far arms extend, correlations between feet and forearms, hands and face, etc.
So, time to watch some naked peeps !!, "Menbung why the PC is filled with naked japanese women!!!", "T-they are for my a-anatomy practice routine!"
I'm sorry I tried to find that awesome page with model ref, but I don't know where I put the link D: