Nothing anyone has said on this thread is undisputed. I was referencing what I read in scientific papers that looked at what physiologically happens under different temperatures without heat acclimation.
Ok fine, but can we talk about the rest of it? Why are we saying carbohydrate use changes with HR due to heat vs due to effort?
I think its important to be careful with what we state as fact without providing references
Because in the study I referenced the power output was kept exactly the same , whilst the ambient temperatures were changed with values of 4C, 11C, 21C, 31C and it was shown that fat oxidation and carbohydrate oxidation changed significantly. Thus effort was not variable.
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It is also important you donāt assume something is fact without first checking if itās been studied in controlled conditions and what the results were.
Iām really struggling to see that for carb oxidation. At 4C it was higher, while the other temps it was basically the same.
For example at 45 minutes, carb oxidation rate of 2.3 grams per minute at a temp of 4C, while the other temps were 1.83g/min, 1.88g/min, 1.82g/min.
Even the Results section notes CHO oxidation rates were roughly the same at 11C, 21C, and 31C. The only outlier was at 4C.
And re: HR, it was the same at 4C, 11C, 21C, and higher on the 31C trial.
Makes sense as 4C to 21C could likely be temps the subjects were acclimated to, but 31C was outside.
The paperās own discussion and conclusion doesnāt appear to support some of the statements you made. Iām not familiar with related research, and canāt judge the methodology, but am left wondering how many 25 year untrained people ride with only shorts, socks and shoes in 4C and 11C weather 
From where I sit, the authors end the article with more questions than answers. Skin and rectal temperature are seemingly the only hard data explaining reduced exercise time at 31C and higher humidity than we have here in NorCal. Would be interested to find studies on trained cyclists.
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So I have a question, I watched the newest Dylan Johnson video with Dr. seiler where they talk about heart rate and power. Now when doing outside rides he mentions following zone 2 power for a certain time and then once you hit your limit (2 hours for example), use heart rate for zone 2.
Is this how I should train zone 2? I used to only go by power but with it being summer, my heart rate gets higher than normal while trying to maintain zone 2. I live in south Florida and our heat index is around 100Ā° by 10-11 am. So im just looking for any advice from this forum.
I rode yesterday for 2 hours zone 2 power, my heart rate after 2 hours was creeping up towards zone 3. So I went by Dr. Seilers advice and followed my heart rate, Iām thinking this is the better way. Just curious from yāall, thanks!!
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I personally pay more attention to HR than power on base endurance rides, that is long rides where Iām keeping it low to mid Z2, no āintervalsā of LT1 or tempo work. Just long āslowā time. What Iām generally looking for is 65-70% maxHR, and not able to nose-breath. So itās a bit of a combination of HR and RPE.
When I start adding things into that a few months into base, Iāll look more at power and HR combined. But for just extending base endurance ride duration, itās HR and RPE for me.
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I also live in Florida and notice the heart rate creep. Most of my rides start around 2-4pm and I will work off HR to stay in zone 2.
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Brother Iām not attacking you, I simply asked a question. I didnāt assume anything. I didnāt state things as fact.
You did Implicitly assume a number of things in your replies. Itāll become obvious if you think about it.
@WindWarrior @GoLongThenGoHome
You focussed your exchange on fat burn, but isnāt the issue fatigue? I know this is perhaps harder to measure, but to me the relevant hypothesis is whether you accumulate more fatigue and/or increase RPE when it is more hot and humid.
Dunno, we didnāt talk about fat oxidation however IIRC that study basically said no correlations between heat and fat ox.
A lot of folks around here embrace the heat and keep on riding/training. Iām part polar bear, but once acclimated its pretty straightforward to continue training in hot & dry conditions of NorCal. After a workout or drop ride cooling down the core is key, Iāve got a pool for that and on a hot evening ride I might use the pool twice before going to bed. FWIW I tend to blow up above 95F / 35C, and yet Iāve seen some people put down big big (for them) 40-min power in 98+F / 37+C conditions. On those evenings almost everyone is sweating bullets at the regroup.
For me the RPE remains aligned with heart rate unless I get dehydrated. But no hypothesis when it comes to fatigue and heat, at the higher temperatures I definitely fatigue much sooner.
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For me personally these things donāt always align and when they donāt align I think what needs to happen is figuring out what your specific training goal is for that ride. Not zone, actual adaptation you are looking for.
I have finished a ride where I am rolling in at high tempo heart rate with mid endurance power.
Which system am I training? Donāt answer so quickly because like everything, it depends. For one thing, what is my RPE? It doesnāt always align with the heart rate. As previously stated, heat, dehydration, caffeine, all sorts of things play a part. When finishing long rides like that, I dont know about you all, but I have been surprised by how high my heart rate is because it doesnāt feel that tough.
So again, what is being trained? Z2 isnt an adaptationā¦ its an ever-moving spectrum used to try to guide you for specific adaptations.
The DJ/Seiler video is a good one, but in a video format it definitely cant get into all of the science which I am sure is huge.
Say one of your goals is training those slow twitch fibers, so you want to do hours in Z2 power to do this. But now youāre halfway through the workout and your heart rate has gone into upper tempo at that same power level. Do you drop power? Why? Now you are not training those slow twitch fibers as what your intention. Yes, I get it that if you have that much cardiac drift you need to work on your engine, but the point still stands.
This is why as I asked my original question.
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