good sketch, I can see how you copied what you saw on the reference for the lighting. But with the reference you used, I think it would be more useful to invent the lighting instead of following what you see for your goal. I have struggled with similar problems a lot also, and I know for myself making sure to focus longer on the gesture and structure before adding the details such as shading can really help.

Nice drawing Kako. It appears with this one that you have more drawn the shadow shapes, which is a great way to show form (and teh muscles light is hitting) but doesn't necessarily distinguish muscles. Was that what you were aiming for?