When going over zone 2

Reading and learning a lot on this forum. Thank you all for that.
I have a question on what happens when you go outside of you desired zone?

So to lose my holiday-extra-kg, I’m now doing rides in zone 2 heart rate. According to Garmin for me that is a heartbeat between 100-146 bpm (max 182). Because I live in a hilly area, even when maintaining a slow speed, my heart rate sometime goes up and out of the desired zone. I then immediately make sure my heart rate goes down again, but this got me wondering the following:

  • if going outside of a zone, how long before the human body switches to a different way of burning fuel?
  • and after going back into the correct zone, how quickly is it burning all those X-mas cookies again?

Because, when this switching between fueling systems is within 2-3minutes, my rides is long enough to not worry about going out of the zone for a short period of time. But when this takes like 15min to return, than a one hour ride, could be useless if I do three little climbs that make my body switch fueling systems.

(Forgive my wording, since I’m still reading up a lot on training)

Some questions

  1. Can you do different loops without these hills?
  2. What gearing do you have on your bike?
  3. How fast do you pedal?

Plus an anecdote.

There is a well known story of Stephen Seiler running in the woods in Norway when he came across a female athlete walking up a hill during one of her training runs. He couldn’t understand this as she could easily run up the hill. She explained she did it to keep her heart rate down on her easy days. It kicked him off on the whole Polarised training research he’s spent so long researching. If the hills are little, is it possible for you to get off and walk them (staying in Z2) until you are fit enough to ride them in Z2?

Hi, thank you for your reply.
My questions is maybe not worded well enough, since I see you’re trying to solve the problem of getting out of Zone 2. Yes, I can ride lower gearing, even walk, but my question is more: How precise should I stay inside Zone 2? Will going out of Zone 2 for 3 - 4 times during a one hour ride for maybe a minute or two mess up my whole zone 2 training?

I think the general answer is no, if it’s just a minute or so, but the experts will be along soon.

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Please do not take this one post as gospel, it is an example.

Today I was a little tired, recovery not excellent, but I thought I would do a little session on the met cart and show you what happens, today.

I will do some different simulations and go a little longer, but this is all I had in me today.

You can see that the fat carb relationship went up.

If I were to have done an endurance session today, it would have been in the range inside the red box.

After the little step test you can see the fat kept dropping, carbs rising, even though I did an interval at 170 watts, after riding at 200w … I will get more examples, but likely you will see a very similar pattern.

I would recommend endurance training in a range, 60-75% max heart rate (for me it is at lower end of that from years of training, even though I am not in great shape.

Heart rate at top of graph, % of max.

Power at bottom.

Fat and carbs in the middle.

If you just stay in the zone, and be really honest, that is best for the rides you are trying to stay in endurance zone.

Get easier gears on the bike if you don’t have them (before gravel bikes many of my athletes had mountain bike gears on their road bike so they could climb hills in endurance zone).

Don’t get too stressed especially in the beginning of your training. Have fun, if you feel a little tired ride easy or rest.

Thank you for your reply.
Looking at the “Fat (Calls/hr)” graph I would derive from it that a human body doesn’t suddenly “switch” its fuel source, but gradually starts burning less fat the higher the heart rate goes. Is that correct?

In this example with me yes.

I will do a few different sessions over the next week or so, then you will be able to see how this changes.

Note I didn’t really go that hard, but I did continue to let power rise, like going slighly harder up a gentle climb.

I went PAST the point at which I would normally do an endurance ride.

Next time I will warm-up to my training zone for endurance and stay there, then we will see what happens in that type of session.

Hello @Gabe

Keep in mind that this is ultimately a simplified explanation for the sake of brevity on the internet.

Our muscle fibers can only contract maximally. This is like one person trying to push a car up a hill. They push as hard as they can and the car rolls backward. 2 people pushing as hard as they can and the car stays in place. 3 pushing as hard as they can and the car moves slowly. 10 people pushing as hard as they can and the car moves up the hill quickly.

As our body is looking to grade force, meaning moving from one workload to another, it does so by recruiting more or fewer muscle fibers. This is ultimately what drives the energy systems within our body.

The initial fibers that are recruited are Type I / Slow twitch fibers. These have many benefits, including their ability to oxidize fat and carbohydrate aerobically. However, they are relatively weak and as we ramp up the force we’re asking of our legs, our legs need to recruit more and more slow twitch fibers, and then more and more fast twitch fibers.

Because fast twitch fibers have fewer mitochondria, they are not able to oxidize fat and carbohydrate in the same manner. Therefore, as we recruit more of these fibers, our substrate utilization (ie Carbs vs. fat) changes as a result.

This process happens quickly. Accelerate from 100 watts to 400 watts, recruit more fast twitch fibers, utilize more carbohydrate. Granted there is a slight delay because we often use creatine phosphate immediately, but the delay is not as long as it would appear in our results. This is because we measure “Fats vs Carbs” based on the oxygen and carbon-dioxide in our breath. This change needs to occur in the muscle, diffuse into the blood, circulate the body, be exhaled in our breath, captured and then averaged by the machine. Therefore, we see a gradual shift over time in our graphs.

My advice to you is that weight loss is more about caloric deficit than training in a “fat-burning” zone. I assume that in Steve’s example above, the top of his zone 2 (in a 5 zone model), is ~200w. Despite this workload burning 273 fat/kcal/hr (which is honestly very good) compared to the 380 fat kcals at 138watts I would choose 200watts because both the stimulus to his aerobic system as well as the higher (832 vs 575) total caloric expenditure.

Regarding staying in zone I’d focus on putting as much time as reasonably possible into your training zone, taking reasonable measures to avoid going harder, but not losing sleep over it at the end of the day.


@robpickels excellent and thank you. I was going to tackle the for weight loss look at calories, not fat calories.

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Thank you for the extensive reply. Much clearer to me now!

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Great response and for fat loss, at least in my experience: its all about weight training. Actually, your weight may even not come down that much, but your body composition changes quite dramatically.