Why is it sooo hard to keep watts up on downhills?

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during my VO2max intervals today i came to some downhills at maybe 4% and with a headwind. i had to go 100% effort just to keep my watts near my targeted zone. as soon as the road leveled off, my preceived exertion was much lower with power in the same zone. then i hit a moderate upgrade and it's a piece of cake to hold the power in target zone. my cadence was relatively the same, slightly lower on the climbs.

anybody want to explain this?

Originally posted by velomanct
during my VO2max intervals today i came to some downhills at maybe 4% and with a headwind. i had to go 100% effort just to keep my watts near my targeted zone. as soon as the road leveled off, my preceived exertion was much lower with power in the same zone. then i hit a moderate upgrade and it's a piece of cake to hold the power in target zone. my cadence was relatively the same, slightly lower on the climbs.

anybody want to explain this?

I believe it is due to changes in the inertial load. To put it simply, our muscles "prefer" it when the pedals move at a certain rate, not only in terms of cadence but also in terms of changes within a single pedal stroke. The latter are generally small because we rarely speed up or slow down all that rapidly, but it seems to be enough to affect our perceived effort even when power is constant (e.g., up or down hill, on the road or on a trainer, solo or drafting behind a motorcycle, on the straights vs. in the turns on a track).

I think this is one of the GREAT things about a powermeter -- it shows you that PE can really mislead you. The bottom line is that you were going as hard downhill as you were the other times -- It just seemed harder because of what acoggan said. You'll find that if you continue to let your Powermeter be your guide, you will retrain your PE to be more accurate...

How did you measured power?