Well, I was looking at the lessons, and found quite a bit of useful material, but not what I was looking for.

Anyone know if there might be any lessons on arms or skeletal structure?

For small budget you could try an anatomy atlas it is good as an art course, you can find it for free on the web
Good luck
Here you learn the basic about using the pencil, the tones, the hatching and after that you aply what you`ve learned on more complex things
Again good luck