BTW TFA doesn't mention the basic idea of backpacks, except by implication: they put the weight on your hips; the shoulders straps are just for alignment.
A properly fitting backpack (with hip belt) is one where the mass of the pack is evenly distributed from the C7 cervical vertebra (the bony part at the base of your neck) down to the iliac crest (top of the hips) where the majority of the weight should sit.