Nice study! It looks appealing and the drawing is pretty strong. - Try to communicate more clearly. The elbow area for example, feels a bit vague, and I can't really tell what's going on. Try to make sure that the viewer sees what you understand and what you want them to see. In order to communicate the forms clearly you might find that you first have to do an extensive underdrawing, making sure that you know where the muscles, bones and tendons are, and where they attach. - When you map out the shading, try to create enclosed shadow shapes, to make it clear what's shadow and what's light. When shading you could follow the approach explained in the beginning of this post https://www.proko.com/s/T1kY. Hope this helps :) Keep up the good work!

Great-looking sketch Marco :) I think the linework is improving and looking more fluid, but I don't have much critique regarding the assignment itself.