This is a study done to work out a problem I was having modeling the back properly. Female back musculature is very subtle. With the male form, the shape is much more distinct and follows the ribs and scapula in a much more predictable way. The female form is more regulated by the fat deposits, and can vary greatly from body to body.
Every lower back is distinctly different, slightly different attachments of the back muscles to the rear of the pelvis create these beautiful lines and shapes, some dimpled, some with hard creases.
This study is done in charcoal and graphite.