What Mother Nature had in mind
When human evolution came to its preliminary end we were hunters and gatherers. We were designed to spend our life in a constant change of rest and light exercise, very much the way we can see our children play. There were rare occasions when high performance was needed, and there were virtually no periods of extended low activity. Our days were filled with a very variable program of running, jumping, climbing, swimming, bending and so forth. Life was a permanent sports event.
But now look at us as of today… There is hardly anybody left who still does what he was designed for. From the perspective of a back, we have a kind of a perverted two class society: those ones who hardly ever move in their offices and cars, and those ones who spent their life with very few restricted movements behind cashiers and assembly lines. And what we were originally supposed to do all day long, we have compressed into a couple of minutes per day in the best case and we call that fitness.
Add to that the fact that thanks to civilization our life span has more than doubled and it comes as no surprise that things do not work well for our back: about 90% of US citizens are plagued by back problems, most of them to a degree that they frequently have to seek treatment.