One of the most confusing parts of figurative drawing for me is the arm, because, from the wrist down, the arm rotates, leaving the muscles, which are not rotating with it, to twist in very interesting ways, while, from the shoulder to the elbow, the rotating ability is quite a bit less. After researching through my resources, I used images in “Artistic Anatomy” by Dr. Paul Richer to draw twelve muscled male arms: four with the hand twisting 360 degrees, while looking at the back of a person, four with the hand twisting around, while looking at the side of the person, and four while looking at the front of a person. I drew the arms, rather than just photocopying them, so that my finger muscles would learn how to draw the arms. After all, practice makes perfect.
I studied the rotations and watched each major muscle shift with each 45 degree twist of the hand. To better explain what I was looking for, I have reproduced the drawings here, and labelled the major muscles so that you can see what I was looking for.
I have also included a photocopy of a diagram indicating the major muscles to help you see where they are in the drawings.