My back hurts, I need to sit down
The forces in your lower back are around double their standing values when you are sitting. When sitting there is reduced lordosis, curvature, in the lower back and the resultant of the forces in the lower spine is pulled to a position where it irritates the associated soft tissues. Nerves in the intervertabral discs respond painfully to structural disruption of the discs and the position we adopt when we sit increases the likelihood of this. Improved muscle strength in the back and abdomen can help to counter the action of the forces that are damaging your discs – damage that is currently beyond the scope of medicine to repair. Sitting more upright helps to pull the vector of the acting forces into more favourable lines of action, similarly to how stronger muscles will. Some academics, rather dryly in my opinion, describe the chair as the most dangerous orthotic device ever developed; under the assumption that for most of the 150,000 years or so of our existence we would not have spent extended periods sitting in one position. Will a new generation who spend more time sitting than previous generations suffer from worse backs from an earlier age? If mine and those of some of my friends are anything to go on then yes, certainly.