(NSFW image references) Learning abdominal/butt muscles anthro anatomy

Hi! This is my first post on derpi forums. I am trying to learn muscles anatomy of human/anthro, to make anthro bodies more lovely. I am curious about second belly bump after buttocks (see link to the image, NSFW) - http://i.imgur.com/6plwAos.jpg
 
I can’t understand what muscles make it. I suspect that it can be “gluteus medius” and “external abdominal oblique”, but not sure which one, or both, or maybe some third muscles in some projections. What is this bump is, and how to draw it properly in different projections?
 
Thanks!

It’s called “the hips”