Poser has a mode that displays skeletons. The skeletons are not perfect, but they are good enough to use as a muscle anatomy learning tool.
I learn best through repetition, and by doing things myself.
I find that I learn muscular anatomy best when I take the time to paint each muscle one-by-one onto a Poser-generated skeleton. This exercise forces me to be accurate about exactly where on each bone each muscle originates and inserts.
I get my information from good reference books. Here's one I recommend:
Human Anatomy for Artists by Eliot Goldfinger.
The Poser skeletons are a little hard to find. Don't look under Content unless you are looking to buy additional content. Poser ships with multiple figures - including skeletons.
You just have to poke around a bit for them. Here's where they are in Poser 6.