Thanks! I sincerely believe it would be a nice addition

Nice! - The shadow values are a bit mixed up with the values in the light. Before adding nuances, try to fill your shadow shapes with an even (really try to make it even) value, so that light and shadow are clearly separated. Value control when shading is really important and becomes more necessary the more information you want to include, like adding halftones after having separated light and shadow for example. As a rule of thumb "the darkest light should be lighter than the lightest dark". You might appreciate this video Mind-Blowing Realistic Shading Tricks Are you familiar with values? If not, you might want to practice doing value studies. You can read about it here https://www.proko.com/s/PX9q. I really like this exercise :) - The upper arm could come toward us more. Right now it feels a bit two dimensional. The proportions look pretty accurate, so it's a matter of subtle changes. I think the most important thing is to make yourself aware that it's coming toward you. Then you do what you can to communicate that. It might help to start with a light underdrawing where you focus on form. I like the process that I've seen Glenn Vilppu use in this Drawing Demo by Glenn Vilppu (the first minutes, and at timestamp 21:00 for example). You start with a flow, then contain it with the major forms, then add the rest. I've found it to give my drawings stronger gesture and three dimensionality. Hope this helps :)

instead of having individual muscles, I tried to simplify the shading a bit