Can’t keep same hr in aerobic workouts

[Axentines]

From the beginning of this year I’ve started building my cycling form using Zwift. I tried «Build me up». But soon realise that long effort at 3-4 power levels completely drain me out. My HR was steady at the beginning of such workout but rose to red zone in the end. I had to reduce difficulty just to finish the workouts!
So I went for 12 weeks «FTP Builder». I found it much easier.
Now I have 5-6 hour of trainings each week. But I’m facing the same problem: can’t keep same HR in aerobic workouts. Yesterday I tried simple (as I thought) TT Tune-Up - Aerobic 12s in Zwift. Everything was fine for 40 minutes: HR zone 2, same power. But in the last 20 min HR goes to zone 4. What’s wrong with me? Poor muscle endurance? Symptoms of health issues (lack of iron)? Poor hydration or nutrition? Or maybe nothing special: I should just keep training?

 

[Cyklopedia]

Your HR will increase with time when you are riding on trainer even holding the same power.
Do you have the same problem outside?

 

[RapDaddyo]

This is totally normal. The problem is with your assumption that HR is a good proxy for intensity of effort. It isn't, either from one day to the next or on the same day. For starters, when you measure heart rate, you are only measuring one part out of two for how the heart responds to intensity of effort. The other part is stroke volume (how much blood is pumped with each beat). Both heart rate and stroke volume respond to changes in intensity of effort, but not necessarily in lock step. So, I'll give you a real-world example. A few years back, in a two-hour hill climb race, I computed my average heart rate and average power by mile. My average HR for the entire 2 hours was about 145, but my average power when my HR was 145 +/- 2 beats per minute varied quite a lot, from a low of 236 to a high of 261. I suggest that you manage your intensity of effort with power, and essentially ignore your heart rate.

 

[Yojimbo_]

It's normal.

Look up cardiac drift.

 

[TrackKingTim]

Hey there! You're absolutely right, it's totally normal. Cardiac drift occurs when your heart rate gradually increases during prolonged exercise, even with a consistent effort level. It happens due to factors like elevated body temperature and dehydration. It can affect performance, so it's important to stay hydrated and manage your effort accordingly. Keeping a close eye on your heart rate and adjusting your training accordingly can help you optimize your performance. Let me know if you have any more questions! Keep on cycling!