The weight of the backpack

A survey of secondary school students found that the average bag weighed about twelve per cent of the student's body weight, and roughly a quarter of bags were over fifteen per cent. The commonly recommended limit is ten.

The effects are not dramatic and they are measurable. Carrying a heavy bag on one shoulder shifts the spine sideways, and the body compensates by changing how it walks. Over months this produces shoulder and lower back pain that students usually attribute to sitting badly. Two straps instead of one removes most of the sideways load, which is why the advice to use both straps is given constantly and ignored constantly, because one strap is faster and looks better.

What the survey found interesting was the composition of the weight. Textbooks accounted for most of it, and a large proportion of the books carried on any given day were not needed that day. Students carried them because checking the timetable each evening took effort and forgetting a book carried a cost.

Several schools fixed this without discussing posture at all. They provided a shelf in each classroom for the books used in that room, so the heavy items never travelled. Average bag weight fell by about a third within a term.

The lesson is familiar: the advice was correct and the shelf worked.