
Of course, the spine is the main landmark, but the trapezius,
deltoid, and latissimus dorsi (lats) muscles along with the scapula,
or shoulder blades, make major landmarks for the mid/upper back as
well. The smaller sacrospinalis muscles of the lower back are also
good points to suggest — even in a low-poly mesh.
Step 1: Shape the Back
For Callisto, broaden her back at the top, flatten it, and then suggest
the scapula and lower sacrospinalis muscles, creating a crease down
the middle for her spine. First, make your work a little easier to see
by hiding the faces you don’t need.
Select the faces of the back, go up to Edit|Select Invert, and hit
the Hide ...