Duke's outfit is pretty basic, when you think about it. He's just a guy in a shirt and jeans, with a little bit of gear. The big, black collar on his shirt is a cool graphic element, though. It almost changes the shirt into a costume. For the chest muscles, it's the same as on the arms. Don't just outline every muscle. Indicate them. Since there's clothing involved, use folds and shadows to indicate those muscles. Less is probably more in this case. A perfect six-pack won't show through a baggy shirt, so just keep that in mind.