Q: Should FTP tests (both original and subsequent) be done in Aero position or in horns? I’m still getting used to Aero position and not overly comfy yet.
A: That’s a really good question, especially when considering how important specificity becomes to event performance. Too many riders want to put in a good time trial, build good fitness to that end, but neglect to spend enough time adapting to the rigors of an exceptionally aerodynamic position. Most of these riders are road racers/stage racers, but I’ve even known some multisport athletes who refuse to spend ample time in the position they’ll have to maintain for hours!
But when it comes to assessment, the goal is to get an accurate snapshot of your fitness – your body’s physiological capabilities – in order to establish training levels and that’s best done in whatever position, cadence, & pacing pattern yield the greatest output. With that said, your power output has to then be “transitioned” to your aero position which takes much time, much practice, some tweaking & tuning of the hardware itself, and as you pointed out, a fair amount of discomfort, at least initially.
So I recommend assessing in your most powerful combination of position, cadence & pacing and then gradually, through trial & error, determining how your positioning affects your power output. Most riders find that, over time, they can adapt their power to their aero position, but many times they’ll find that their position is too aggressively aerodynamic and no amount of practice is going to yield their highest power output in a posture that’s too severe.
I might as well point out the dilemma between a less powerful but more slippery position which thereby yields faster times on particular courses, but this only further illustrates the need for trial & error.
So assess comfortably – relative term, right? – transition your power to an aero position, and spend time sussing out a proper balance between power output and aerodynamics.