Recently, a few people were discussing this on the wattage forum, but I don't feel that any conclusive answers came out of it.
Basically, why is it so much easier to create watts on an uphill? Is it the lower cadence (which, suits me better)? Is it the ability to engage the hamstrings more effectively in that lower cadence? Am I just lacking high cadence training? Is it the fact that on a downhill your power with each pedal stroke needs to increase within that pedal stroke to keep that wattage up because the resistance is, in effect, going away from you (as your speed increases)? Where as on an uphill, you're not accelerating speed-wise, so your pedal strokes can be met by an equal amount of gravity?
I think it's safe to say that I am more strength oriented, and less cardio-fit (genetically, not just from a training or lack of standpoint), but I'm not sure if that plays a role.
This question came up for me as I'm trying to do 5x5 minute intervals at a set wattage. When doing 6x3's at that wattage two weeks ago, I'm not at my limit (although it hurts). However, now that I'm playing with my upper boundaries, if I hit ANY downhill or whatever in that 5 minute segment, and attempt to maintain my wattage, the pain and cardio taxing increase past my limit - I'm screwed. By set #4 today, I was toast.
I'm aware that there are probably drills I can do to increase efficiency at higher cadences, but my gut tells me that there is something else going on here. It's related to why WaterRockets does his WR Intervals (or just the 1 minute test) on an uphill. Why?