Hogarth can be pretty tough. I'd set his stuff aside for now and go with someone like George Bridgeman, he simplifies things into more practical units a lot more. Another approach that was recommended to me is if you have scans of anatomy book pages, print some of them out and use them like a colouring book, shading in the different muscles with coloured pencils or Col-Erase.
Don't worry too hard about getting into too much detail, like every muscle of the hand or whatever. As an artist, you're mostly interested in how the skeleton moves (range of motion, where things bend, static lengths, etc. - Loomis' mannakin forms is great for this) and visible muscle groups. So unless you're doing a heavily defined character, you only need the general shape of the deltoid rather than the separate heads of it.
If you can find a life drawing group in your area, definitely go so you can see these things in person. It'll help a lot.
