Can’t keep same HR in aerobic workouts


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 intervals at 3-4 power levels completely drain me out. My HR was steady during first interval but rose to red zone in the end of last 3 interval. 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 overheating? Or maybe nothing special: I should just keep training?

If you lower your ftp in game the workouts will be easier!

It’s hard to say if anything is ‘wrong’ without more information. But, personally, I would expect that your HR would be higher toward the back end of the workout as your body is tiring and doesn’t have the time to recover during the workout. Part of the point of the workouts is to push your body past its present capabilities, so your body having to work harder makes sense. Unless you are having other issues along with the increased HR (lightheadedness, difficulty breathing, etc.) I would say to just keep going. (Keep in mind that I am NOT a doctor.)

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You’re not (yet) a highly trained person. For a beginner, 40 minutes are on the boundary of some stress factors, like depleted Glycogen, dehydration, heat, neuromuscular fatigue, etc. etc.

No one will be able to maintain the same heart rate at the same power forever. A transition from high 2 to low 4 is not an issue. So said, I agree with Nigel, keep going.

I’m not a doctor, too, just a 65-year old guy with more than 40-year long experience in training.

Hi. Paolo


To add, heart rate training is easy but not so reliable on the end. You cannot guarantee that your Monday’s zone 2 equals Tuesday’s one.

Your HR is bound to go up the longer you do the intervals. For example, typically HR on interval 5 is going to be higher than on interval 1.

What’s your cooling situation? How many fans? AC? This is super important when doing indoor training. W/out adequate cooling, you will see you HR be extremely high for a given effort when compared to outdoors.

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Heart rate drift is completely normal and varies depending on your level of fitness.
I did the 3R 100km 2.5W/kg ride this AM and started with HR holding steady 115-120 but then by the end I was in the low 130’s with no change in power.

Thanks to all for your replies!

I can’t tell that I’m completely a beginner in cycling. But I’ve never race before and never bother to do a structured workouts. I’ve competed 12 weeks FTP Builder and thought that I have enough muscle endurance to start TT Tune-Up training plan. Maybe, I was wrong :slight_smile:

I have one fan that blows directly at me. Yesterday I tried to take a long brake (hitting «pause») during workout so I could cool down. This didn’t work as I expected: I had three intervals in 3 power zone. My HR was 130 after first one, 136 after second and 144 at the end of the last one.

That gradual increase is normal as far as I’m concerned.

Hi Igor,
you might find that running the fan slightly lower than yourself and off to one side may increase the surface area that your fan is able to cool.

That or run more fans… I think I’m on 3 fans now.

“Ride On”