Back muscles are actually much more difficult to learn (and to draw) than muscles at the front of the torso. This is because they are in two layers that interact with each other in a not overtly muscular person. So while musculus trapezius and musculus latissimus dorsi are the dominant ones, the underlying muscles are important too. And the muscles around the scapula are a nightmare.

©Charly, all rights reserved. Click for full size.

I do not actually have any interesting story to accompany this picture. So I will only let you with the obligatory “lift with your legs, not with your back” warning. Back muscles around the spine are not optimised for load bearing and lifting heavy objects using your back muscles is a sure recipe for health problems later on.