There is an old adage that says carring 1 pound on your feet requires as much energy as 6 pounds on your back.l suppose this is because your feet stop, get lifted and accelerated with every step. Personally, it does not seem that much different to me. For example with my 4 pound snowshoes on my feet or on my back l still travel at about the same speed.

l think l had heard 10. It is much harder to carry them on your feet instead of your back. Put on some ankle weights and go for a run and then carry the ankle weights in your pack and run and you will see a big difference.

l dont know how much extra it would be on your back, but it is definetely true that it is harder for more weight to be on your feet then on your back

It makes no difference in the long run in terms of energy, but you do feel it in your legs if you spend five or six hours on a long trek into the mountains with a pair of heavy, damp semi rigids, compared to a pair of old light runners. Not sure why, think it might in part be the more awkward step.