This is getting borderline philosophical. Empiricists would argue that only things that can be perceived exist. Realists, though, would argue that perceptions are based on real existing phenomena, but that the perceptions might be distorted. I would argue that there are two things going on here: real effort, and perceptions of that effort. Given that both can be measured, why has the research only looked at perceived effort?
Spot on. Because the answers everybody has given have, of course, been about the reasons for real increase in effort.
I'm sitting in my lounge typing on here, but I perceive that I'm putting a lot of effort into it. Will I lose weight and get fitter?
...pretty much sums it up.
I sometimes feel like I'm running faster and more freely while at others I feel like I'm plodding. My watch, however, tells me I'm running at exactly the same pace. The real question is, since it's not a physical thing what is it that psychologically causes the difference in perception? Because if there was an answer to that, running could potentially feel more enjoyable more of the time.