Well, if we understand what lies beneath the surface we can make some informed decisions on how to represent what we see on the surface. We use anatomical landmarks to help understand and make sense of what it is we are looking at. In turn, we can then make creative decisions on how we want to represent those ideas. When we start to learn Anatomy it can be quite intimidating but it's not essential to know all the names of all the muscles. Knowing some of the major groups and how they relate to each other will go a long way and over time your knowledge will increase and a certain familiarity will start to take hold.
If we break it down into sections, we can make it manageable and develop our understanding in an incremental way without having to get overwhelmed by the whole.
Get out that Anatomy book!
This is a good place to start. Open the anatomy book sitting on your shelf and have it next to you while you draw. If something that you're seeing on the figure needs clarification then you have reference easily at hand. I'll do a book list in a future post where we can look at just a few of what are probably thousands of books on the subject at this point. Another option is obviously google images, but it's a bit more chaotic.