No 6 year old is lugging around a backpack full books; these are teens we're talking about.

5 kg is only a couple of textbooks. Teens need to pick what books to take home. Obviously digital is free weight, but physical textbooks have a real weight cost, I don’t want my kid going through what I had to go through.

That paper also finds problems with sitting:

> However, when the child begins to attend school, their time spent in a sitting position is extended, which can result in disorders of posturogenesis.

> However, there is still no clear information about the impact that a school backpack has on the formation of spinal curvature in the sagittal plane in school children.

The next sentence following the stats quoted from the link.

Many experts will tell you walking 30 min a day is "enough exercise" too... I'd be very suspicious of these studies