Why is that? I’ve noticed the same thing and figured I was an anamoly.
My biased and unscientific thinking - blood needs to work against gravity to go up to brain or other parts…
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It should improve when trained on regular bases.
Higher rpm will always have some HR raise because for the same power, you will use more cardiovascular fitness (oxygen) and less muscular power. That’s the advantage of high cadence. You save on muscle fatigue and simply use a bit more abundantly available oxygen (if cardiovascular fitness is high enough off course).
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What I wonder is what to do with small hills (10~20m at most I guess? No idea, I’ve never measured them). Should I try to get over those at most at high-end-Z2 or is it fine to throw the anaerobic motor at it for half a minute?
Since @MedTechCD is here … wondering whether keeping an eye on AlphaHRV is also a good way to see whether one hasn’t abandoned the Z2 effect due to over-exertion. 
I think you’re fine to go into Z3 a bit, but I don’t have any evidence for that. It’s usually what I do on punchy hills when I’m a long endurance ride. What you do may be influenced by what you’ve done and plan to do in the few days surrounding your ride.
I utilized VO2 max training into my Z2 efforts, but always at the very end. For example I will conduct a 2 hour ride, and the last 10 minutes i will conduct some 30/30’s to give myself a boost at the end, followed by a cooldown.
I figured this the best way to mix it up a bit without compromising the intent of the z2 effort.
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I experimented with this yesterday, in which I’d eaten normally ahead of time but took on no fuel during the ride.
Positions 1, 2: 4min intervals of lower VO2 power (maybe my 8min power… I’m not totally sure what my curve is these days).
Position 3: I switched the erg setting from 235W to 240W.
Position 4: A return to my “baseline” ratio of power to HR.
Position 5: A brief period of standing up due to some discomfort. After this, my ratio of power to HR started to drift lower, which I suspect was energy availability related. Hydration was acceptable.
Position 6: 2x30sec above VO2 just for curiosity.
You can see a gradual decrease in HR from 22min (when VO2 ended) to about 40 minutes later, where it remained constant for the next 40min or so. How this is related to the time it takes to return to Z2 metabolic process, I’m not sure, but thought people might want to see.
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That’s an interesting diagram, and confirms something from another thread: even at “Fat Max”, these athletes were getting 76% of their fuel calories from carbohydrate, burning about 120g / hr.
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Here are a couple of examples one short interval ride & run where I have DFAa1 data. The ride would suggest that I probably would have stayed well within Z2 & so primarily fat burning (assuming a 0.75 DFAa1 threshold is appropriate).
However the run that I did probably had intervals that were a bit too long to keep me in Z2, or at least primarily fat burning perhaps. (On a TM intervals less than 1min are a bit of waste of time due to time TM takes to get up to speed). The HR/DFAa1 zone & fat or glucose burning will depend on the frequency & intensity of the hard efforts. Initially on the run I was back above 0.75 a1 fairly quickly but then for the last two where I pushed harder then the recovey was not so quick especially after the last one. So it would take a bit of experimentation with interval durations & intensities. But also if using RRa1 instead of DFAa1 & so including the breathing rate/intensity (which is probably more applicable to running for me as I tend to mouth breathe at slower intensities than I would for cycling) then I suspect that I would likely have switched to primarily burning glucose based on that trace. But everyone is individual & so really everybody would need to do their own personalised testing.
AI Endurance Anaerobic Run (1min intervals, 3min recoveries). With AIE’s DFAa1 summary stats for the run below.
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Another ride from today with very short intervals. Again DFAa1 looks good for below AeT but RRa1 is slightly more questionable as I remained sufficiently under AeT to always be in Z2. But I doubt whether the potential dips into Z3 based on RRa1 would have been sufficient to switch me from fat burning to glucose burning. But wiser people might chip in with other ideas perhaps.
I can post a handful of quality studies (at least one posted earlier) showing “primarily burning fat” is unlikely happening and a myth. The exception is if you are in ketosis and not taking on carbs.
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