While I don't think it's explicitly mentioned in the book, it seems unrealistic to stretchsit for very long without good abdominal tone (not sure exactly which muscles).
Without good tone, it seems either the upper back has to arch back to compensate for gravity (leading to pain) or, if one maintains a straight upper back, the neck and shoulders have to work overtime to keep one from collapsing forward. After months of trying, I can never seem to get through a whole work day without either upper-back-pain or muscle soreness.
The only problem with good abdominal tone is it makes it harder to breathe, since it's unrealistic to breathe through our ribcage wall if we haven't been doing that our entire lives.
Has anyone else been through this cycle? How did you escape it?