Oh wow, I didn´t know that writing critiques was that offcial here, but I think I´d like to stick to the comment section haha. I wanted to say something about your art anyway so..

So first I like your body proposition as well as the facial. I like the lighting dash in the hair as well as the shading on the arm to the hand. Also the tank and its bulges are pretty accurate. I don´t whether its your art style, but I think the eyes seem too round, which makes Mark look less strong, but that´s fine I think. You may consider to make his triceps more round, as well as to add more shading or light reflections on his upper arm to increase the body form of the muscles. Maybe I´m overthinking, so yeah.. Sorry if that sounded too harsh