The set of muscles that we commonly refer to as "abs" actually sit on top of another flatter layer (see insets in illustration) so they form a visible column of muscle. MakeHuman only hints at that and I think adding more of that contour gives the stomach a significantly more realistic look, especially on more muscular bodies.
I created a target that pushes the abs out from the torso to form a column and uploaded it to user contributed assets. http://www.makehumancommunity.org/target/abdominal_column.html