I’m pretty similar. A 6 hour Z2 ride yesterday. Mostly sat around 68% max with a cap of 76% max on uphills
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Neither does partially quoting people. 
This weather in the UK is a great example of how HR does not reflect effort or output consistently. But you’re making this a debate about HR vs power, for which I don’t think either of us have anything new to say.
My points were;
- Base your aerobic zone HR around the last half of Pettit
- Work on outdoor pacing by power
Interesting discussion, but I would really be interested in the training impulse and physiological toll on your body when pacing HR vs power.
For instance, if I am doing Z2 ride by heart rate. it is 70 degrees (F) andI average 220 watts and 110 bpm.
A week later, its 90 degrees out. I average the same watts but my heart rate is 130 average.
Besides sweating a bit more on the hot ride, my RPE is the same.
Is this taking away from the Z2 stuff? Am I not getting the same Z2 benefits or am I causing more fatigue than usual because of the increased heart rate? I know the easy answer is: of course dummy, youre in Z3 now!
But I don’t think its that simple. When I am fresh my heart rate is a little higher but my RPE is lower as well. Doesn’t mean my zones have changed.
I guess it boils down to the training by HR vs training by power discussion.
Seiler often talks about the difference between internal and external load, power is external load and heart rate a measurement of internal load. I think he recommends going by heart rate or feel (are you able to easily hold a conversation) rather than going by external load (ie power) for zone 2
Your muscle recruitment isn’t the same, your oxygen consumption isn’t the same, your fat oxidation isn’t the same, your CHO isn’t the same, your anaerobic contribution is higher the hotter it is, for the same power. If your Z2 power target is nowhere near top of Z2 (in the conditions you determined that) you might be okay, otherwise you’ll definitely be into Z3 sooner or latter.
It’s why I prefer HR for the Z2 because it is taking into account the extra physiological costs due to temperature etc. if reflects how hard you are actually working to maintain that power under the conditions you find yourself in.
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I can’t imagine my HR being 20bpm higher, with increased heat and sweating, and feeling like my RPE is equal.
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Ah but it doesn’t - and it isn’t a measure of how heard you can go.
It’s affected by the weather and you can take it into account but ultimately for your best performance you still need to be guiding by power.
if you are unable to tell it’s too hot without HR telling you it’s too hot, then sure. But if you are saying it’s too hot, I’m gonna stay with 150bpm or whatever then yes - we are in disagreement. HR doesn’t describe the output you are capable of, it is merely one factor.
“Affected by” is by no means “takes account of”.
As I said earlier I suspect we won’t make headway as neither of us are bringing anything new to the table in the HR vs Power debate, and it’s not the reason I joined this thread.
I’ll leave it there 
References?
After some heat acclimation, I’m doing zone2 rides at same power:HR now when temps are 84F/29C vs months ago at 60F/16C. This is a good paper on the topic of heat acclimation: https://onlinelibrary.wiley.com/doi/10.1111/sms.12408
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I have no references but I put out much higher power in the morning at 75F that in the afternoon at 95F with the same HR.
sure, but have you tried to acclimate to the heat? There is a classic 2015 study where they did two 43km time trials in Denmark, where the cyclists had been training where max temps never exceeded 10C / 50F from November to March. About 14 hours/week. Then they went to Qatar and did 3 time trials in average temps of 36C / 97F. First day power was down about 16%. Then they spent two weeks being outside a minimum of 4 hours a day. The fifth day power was down 8%. The 13th day power was only down 3% but they finished basically same time of 64 minutes (although about 40 seconds faster in heat versus in cold, due to reduced air pressure).
Average power and hr:
- cold: 304W / 166bpm
- on 13th day of training in heat: 296W / 172bpm
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Which is the big clue, you don’t get that chance in the UK.
HR increase in relation to increasing levels of cooling requirements doesn’t mean that you have the same metabolic cost.
I haven’t done the required reading for quite some time, but going by memory, that’s the risk of pacing by HR. You can be underselling physiologic adaptations by reducing your effort.
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Skiba’s new book Scientific Training for Endurance Athletes has this analogy: Your heart is a pump, connected to a series of pipes (blood vessels). The pipes open up when you exercise, reducing the pressure, and your heart responds by pumping harder and faster. Heat causes your body to open up tiny pipes (blood vessels) in the skin to shed heat, and that also reduces pressure and causes the heart to pump harder and faster. And that is before considering fluid related (plasma volume?) pressure losses from sweat, respiration, and urination.
The net result is the heart beats faster when exposed to heat stress. With continuous exposure to heat, It takes about a week to normalize, and 2-3 weeks to bring your body nearly back to the same state as exercising in cool weather.
Heat acclimation will increase aerobic capacity / VO2max about 10%, however even after 2+ weeks of acclimation you don’t completely return to baseline as seen in that 43km time trial mentioned above.
Metabolic adaptations to leg muscles include reduced use of glycogen, an increase in lactate threshold, and reduced lactate in muscles.
A single really hot day? I think the pump analogy explains the majority of heart rate increase due to reduced blood flow at muscles due to an increase in blood flow at the skin. So the heart compensates by beating faster. There is also some additional stress on the body, due to elevated temperature in muscles after exercising and perhaps a few other things that I don’t recall from that earlier “Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports” article I posted.
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I don’t think everything you said here is undisputable fact. Simply having a higher heart rate doesn’t necessarily change these things. If it did, we could just ride with fans off on the trainer during zone 2 rides and get Z4 benefits.
Yes, and carrying fatigue will also do the same thing.
If I am carrying lots of fatigue and cant get my heart rate up, it doesn’t mean I am now training Z2 instead of Z4.
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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 don’t assume something is fact without first checking if it’s been studied in controlled conditions and what the results were.