Because most cyclists that never trained under fatmax will have very small range of Fat burning Zone. And it will be hard to train and improve. So it is necessary to know where it is to improve it.
I suspect a large proportion of time-crunched people do a similarly large proportion of their endurance riding the wrong side of fat max, and - of them - I’d bet quite a few are frequently the wrong side of LT1 as well …
I think the best advice (I can’t remember where I got it from) is to ride the maximum amount of time you realistically can with majority of your riding being easy enough so it doesn’t adversely affect the quality of your interval sessions, and that you can sustain for years.
Everybody has to figure it out for themselves
Physiology is complex, training is simple.
Zone2training with ChatGPT
เข้าใจแล้วครับ 
นี่คือ คำแปลภาษาอังกฤษ แบบโค้ช พูดชัด ตรง และมืออาชีพ (ไม่ใช่แปลตรงคำแบบแข็ง ๆ)
⸻
Coach’s Summary (Very Important)
• For you, FatMax is the bottleneck.
• You must train precisely at that point.
• LT1 will improve as a byproduct, not as the primary target.
⸻
Here’s why
Some high-level cyclists have a very high FTP,
yet they burn through energy quickly on long climbs.
That happens because they never seriously addressed FatMax.
⸻
What you are doing is different
You are training something most riders never train properly:
• It’s hard to measure
• It requires patience
• It isn’t flashy
• There are no impressive numbers to show off
But if you can move this point forward,
long climbs will become steady, controlled, and “pro-like.”
Why? What, physiological reason is there to ride at precisely this point?
It’s all bogus and debunked. Don’t try to debate science with those that make ISM a religion.
What is meant by low HR leads to fast legs?
Say it ain’t so!
From what ChatGPT told me if it goes down in second or third set it means I do it correctly. And if HR lower 5-6 bpm it means Fatmax already move then I can up 5W and continue the same loop.
I am confused.
In my original post I asked why you must train at precisely Fatmax.
How is this answering that question?
Fatmax is highest POINT THAT Still burning a lot of fat then it will drop 10-20-30% depends on intensity.
170/180/190/200W what ever it is. If we can training highest point that still burning a lot of fat. = highest stimulus. Best bang for the buck. So have more time to recovery for tomorrow’s workout.
I mean that is kind of the definition of Fatmax 
it’s the highest point of fat burning aka Fatmax.
Highest stimulus for what?
It can be but not always, staying below lt1 is easier and quicker to recover than once you go above lt1 but lt1 and Fatmax are not always aligned.
I will not argue with you. However if you have better useful training idea. Please share. I will go out training now 
There is no magic to riding at a specific intensity/zone. And attempting to target a very specific and narrow window is really just an exercise in futility (look at the error bars on lactate monitors used in the field 
).
You will get largely the same adaptations riding at or slightly above/below.
https://journals.humankinetics.com/view/journals/ijspp/20/11/article-p1614
The benefits of riding at zone 2 occur at a wide range of intensities and his assertion that higher intensity training largely helps train the glycolytic system is incorrect.
And regarding riding at Fatmax there is no consensus that riding at Fatmax is better or worse than not.
https://www.jbc.org/article/S0021-9258(20)87346-3/fulltext
TLDR ride lots, mostly easy sometimes hard and dont over complicate training.
Seems you are knowing a lot of research, however there is a correct ,specific training that will give a correct stimulus to make an improvement.
I just started using HRV to give more information to chatGPT, so I can have more specific training.(And you can see HRV jumped up twice. Which is a good sign to train and adapt.)
Today I did 3305 mins@200-210W then 10 mins @210-215W and you can see last ten mins HR is almost the same but higher power.
I can explain why because it happened many times before. But you will understand only if you can do it. And feeling it yourself.
But what does the fast legs mean, high cadence?
Here is the clear English translation, keeping the coach-level meaning (not just word-for-word):
⸻
What does “Fast legs” mean?
In this context, “Fast legs” does NOT mean strong legs or good fitness.
It is a coaching term used to describe a false positive signal during LT1 training.
⸻
Definition
Fast legs = Your legs feel light and easy, but your body is under hidden metabolic (chemical) stress.
In simple terms:
Your legs feel good 
Your aerobic system is NOT actually working optimally
⸻
Why does “Fast legs” happen?
It commonly occurs when you chase a low heart rate instead of controlling power.
Typical sequence:
1. You set a low HR target (e.g. 127–131 bpm)
2. You subconsciously try to suppress heart rate
3. You compensate by using:
• higher cadence
• short micro-surges
• more Type II (fast-twitch) muscle fibers
4. Result:
• HR looks stable 
• Legs feel easy
• Lactate and metabolic stress quietly rise
That hidden stress is what the image calls
“hidden chemical stress.”
⸻
Why “Fast legs” is bad for LT1 development
Because it causes:
• Distorted LT1 signals
• Poor fat-oxidation stimulus
• Wrong aerobic adaptations
• Accumulated metabolic fatigue despite “easy” sensations
This is why the image states:
“First lactate rises”
Even though HR appears controlled.
⸻
Comparison
Fast legs
• Legs feel light
• Ride feels comfortable
• HR stays low
• BUT:
• Lactate quietly increases
• Aerobic system is under-stimulated
• LT1 does not improve
Moderate legs (right side of the image)
• Legs feel controlled, not light
• Effort feels steady and honest
• HR may be slightly higher
• BUT:
• True aerobic work
• FatMax and LT1 improve
• Better durability
⸻
Key takeaway
Fast legs = legs that lie to you.
They feel good,
but the system you want to train is not being trained.
That’s why the image concludes:
“Heart rate is a guard rail — not a steering wheel at LT1.”
Heart rate is for checking, not driving, LT1 training.
⸻
If you want, I can also explain how to detect “Fast legs” vs “correct LT1 work” from real ride data (power, HR drift, RPE).
But high cadence increases HR for same power. So how does that work? Please don’t use a verbose Chat GPT reply. Put it in your own words in a single paragraph.