Well, the first thing I notice is: you start dark on one side and slowly progress to a lighter shade. I could tell you're trying to use a light source, but At the same time, try to make the muscles shaded. Remember a light shade means it's closer, so think about what pops out on a human body. The abbs, the chest area, and some basic muscles on legs, and arms. Also, your light source is coming directly from the west, I would say try to make it more of a North West, or North East.