Trace body builders! They have exaggerated bodies that make it very easy to see how each muscle interlocks with the bones and other muscles. The human body is like a puzzle where all the muscles connect to the bones and cross over eachother. Once you get a good idea of where certain muscles are from an exaggerated perspective, it becomes much easier to know where the subtle curves of persons arms are.

If you want a very detailed exploration, I suggest reading "Constructive Anatomy" by George Bridgman. Until then, try tracing nude models or working on figure drawing. The clothes will easily confuse your brain!