Digging up this old thread which is fascinating but sadly inconclusive. I particularly appreciate the lactate testing, as it sometimes feels like we outdoor-dominant riders are being gaslit by the folks who claim we just need more fans or mental fortitude.
Here are two tests that I hope someone who is experiencing less power output indoors will be able to run to shed some more light on this issue:
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Use their trainer outdoors in very low temps, such as <30F. This should immediately prove or dispel the temperature hypothesis. (Happily I do not live somewhere where it gets cold enough to try this.)
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Use a system such as the e-flex motion or rocker plates to add movement to the trainer experience. I have my doubts about this as I’m pretty sure the marketing copy for these products would be full of claims such as “boost FTP by 10%!” if there were even a modicum of reality to this line of thinking. But I don’t know what the difference could be if it’s not either muscle recruitment or temperature. (I will probably eventually break down and run this test myself.)
And lastly, one more reason I don’t think it’s down to temperature is that my outdoor FTP doesn’t seem to care if it’s 40F (as long as I’m wearing throat protection) or 100F (so long as I’m acclimated), and I produce my best numbers on climbs where I’m getting all of 8mph of relative windspeed.