/ic/ - Artwork/Critique

Question: I'm currently learning anatomy and heavily struggling with the anatomy of the arm and especially the forearm. I want to be able to draw (pic related) muscular arms from imagination, not just drumsticks. The arm can be lifted, supinated, pronated, the hand can be flexed in and out and every time it's shape changes, much moreso on a muscular person. How should I go about about learning it's underlying anatomy? Are there any good videos? Any advice is welcome.