The musculature of the human body is quite complicated, but there are many good books showing the arrangement of all the muscles and how they lie across each other and bind around the bone structure, giving us a much clearer idea of how the human body gets its shape.
As artists, our primary interest is in the structure of the muscles on the surface. There are two types of muscles that establish the main shape of the body and they are either striated, or more like smooth cladding. The large muscles are the most useful ones for you to know about, and once you have familiarized yourself with these it is only really necessary to investigate the deeper muscle structures for your own interest. If you can remember the major muscles, that is good enough for the purposes of drawing figures.
Proportions of the human body- human musculature side view and back view
