Well, I can't speak with any empirical evidence, but I can tell you that I *feel* like I've worked harder when I've carried a load. Depending on the weather, what I have to take to work, and day of the week, my load can vary by 10 or 15 pounds, I'd guess. Mondays are usually the heaviest days because I'm taking in all my snacks for the week, plus whatever books I've been reading over the weekend, plus my regular change of clothes, toiletries, etc. My times on Mondays are usually somewhat longer than those on Tuesday.