I feel you could use black as a shade rather than an outline. It's a pretty solid sprite but I do think it definitely could be shaded better - such as like highlighting muscles in the arms and stuff. @Taki's pretty good at that stuff. You could shoot him a PM.
As for clothes, you need to figure out the anker points and let wrinkles fall from them like strings. Simply said. What you have now is wrikles coming and going in parallel direction, which is totally unnatural. The pants are much better in that regard. As for shading, ehhh, it's a very common pose, you can find a reference for those muscles really quick, so i don't know what to tell you. Colorwise you need some work. It's way too dark, could use a highlight rework to make the muscles stand out more and a backlight.
