Four hundred years ago Sir Isaac Newton pointed out that Work equals Force times Distance. Work is the energy needed to carry a heavy load over the landscape. The weight of the load is the Force against gravity. Each step forward requires a small Distance upward and here is where the work is done. Indeed the old-as-dirt adage that "a pound on your feet equals five on your back" follows because on average feet move, say, five inches upward with each step forward. Meanwhile the pack moves but an inch upward in the process. Thus the Work needed to move one pound of footwear one step is five times the Work needed to move one pound of pack weight, because the feet are lifted five times the Distance the pack is lifted. At least that's how I figure it. I understand that work is only done when there is a force to resist it but will weight on you feet really wear you out on the end of a hike as much as fives times that weight in your pack?