Some arms desuuuuuu...
Muscly man arms that is! I need to draw more feminine arms >.> I draw arms a lot don't I?
Just mentioning the red outlined drawing aren't muscle structure, they are just the shapes I use to plan the arm. Like I said in my other tips sheets, dividing anatomy up into shapes makes it much easier to draw consistently
Using a reference doesn't hurt either. Studying from real life is the best way to set a foundation for anatomy, then simplifying it into what ever style you like.