What every backpack needs, it seems, is bungees. Carefully springing a backpack reduces the vertical travel (as percieved by your body) to about one-fifth the directly-connected values. Puts a spring into your step, that does.
For example, a person able to carry 48 pounds in a static backpack can carry 60 pounds in a bungee backpack, and can also run more comfortably.
Lightning Packs LLC is now working on reducing the total weight of the backpack, & developing a light-weight “daypack” version.