Friel’s AE2 - Power and HR Relationship

For anyone who has read Friel’s Bible or employs his workouts, I’m planning on topping up a TR Low Volume plan with his AE2 workout. Pretty simple directions; ride at your aerobic threshold HR for the duration of the workout. AeT HR estimated at 30bpm below your Lactate Threshold HR, +- 2bpm.

The question: when I ride at this HR, I am solidly in my Zone 3 power, about 80% FTP. Does this not seem too high to be my go-to endurance ride? Friel notes that this workout should be done year round to build and then maintain aerobic fitness. Anyone else’s power this high when riding at your AeT HR?

How have you estimated your LTHR? How recently have you tested it?

Either way, you’re overthinking it. The intent of Friel’s workout is long and steady in zone 2. You can do that by power, HR, or even RPE. Ask a bunch of coaches about which way to do that workout (power, HR, or RPE) and you’ll get a bunch of answers… but you should not be riding in Zone 3 by power.


Honestly I think this is wicked common and I see it every year especially starting a big base again. Your aerobic fitness is high compared to muscular fitness. That and sometimes zones don’t necessarily align perfectly.


As Landis noted, it is very common for zones to not align correctly….and I don’t agree with Friel’s calculation for AeT HR. Subtracting a defined number from a variable one will result in different HR values. IOW, training zones are defined by percentages…either of FTP or Max HR. A set number (in this case 30 bpm) will yield different results if your threshold HR is 160 (~19% of threshold) vs. 180 (~16%).

My suggestion is to pick one metric to train by and stick with it….don’t look for alignment. Personally, I would default to Power, but some prefer HR.


Great advice. If I’m trying to ride as close to the aerobic threshold as possible, where does that typically land power wise?

It is not possible to calculate it yet from power or by HR only (you can use DFA Alpha1 that apparently works well) It is around rpe 5/10, the point where your breathing changes slightly or you feel that you are started to going into tempo zone and you can feel that differently day by day. RPE is best for me to feel the point where you are working hard but you know that you can do this for longer time. Everyone will have different point but it lands around 0.7-0.8 of your FTP.

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If you can figure out where your LT1 is on any given day, you’ve discovered the endurance unicorn and are destined for millions.

I ride around 65% of FTP and that’s good enough. There’s really not much additional benefit to your aerobic metabolism to ride higher in zone 2, so I aim for the low or middle portion and focus on time in zone.


Generally the way I do these workouts is negative split at or slightly below perceived LT1. Start low. I might start at 60% for the first hour, 63% then up to 65%+ over the course of the next two hours or more. I’ll watch HR and RPE throughout, and if either gets higher than I want, I’ll back it off.

I look at HR afterwards for decoupling purposes.


If you want to open this particular can of worms:

A lot about Z2 and riding near lt1


In the latest edition of the book, the info is spread across chapters 4 and 5. It starts with a discussion AeT:

  • RPE of 3-4 out of 10
  • lactate testing is most accurate way to determine
  • gas exchange is 2nd best
  • next is using rough estimate of 65% of HRmax
  • another rough estimate is 20 to 40 bpm below your threshold HR
  • an even simpler rough estimate is 30bpm below your threshold HR, which he thinks seems to work well for average fitness rider

Power and HR zone alignment:

  • power and HR zones don’t always agree
  • don’t be concerned, its common and expected

The functional AeT test:

  • following a warmup, ride for 1-3 hours (based on duration of target event)
  • ride at AeT HR, which to quote Friel “is LIKELY to occur at 20 to 40 beats per minute below your FTHR” (his words, my emphasis), or you can use FTHR minus 30bpm

There is a lot of wiggle room on those rough estimates.

HR is really personal, you need to figure out what works for you. This is what I’ve done for 5 years… my Friel zone2 HR is 131 to 142bpm (Coggan zone2 is 110-134bpm). My “all day HR” from doing 6-16 hour rides fall into that range. Eyeballing it, my average all-day is around 136-138bpm. After a season break I train by HR at 136-138bpm, and if feeling tired I’ll dial it down to 128-130bpm. After a 4 week block of endurance I’ll switch to training by power and my zone2 power usually results in zone2 HR which for zone2 workout at 70-72% FTP is 136-138bpm right in the middle of Friel but above Coggan (so I ignore Coggan HR zones).

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This certainly sheds more light on the issue. Thank you!

and some info from Matt Fitzgerald on a lower boundary for easy aerobic work:

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Interesting that in Fitzgerald’s article he notes not to dip below 68% of HR max but in Friel’s latest edition, as per your summary, he states AeT is estimated at 65% of HR max. Is that a typo?

I dunno. Rough estimates. My recommendation - don’t be lulled into a false sense of precision. Taking it one step further, to illustrate my point. Is it better to train just below AeT? Is it better to train right at AeT? Is it better to train above AeT? Should I be doing over/unders at AeT? I haven’t seen any research that definitively answers those questions.

To Kurt’s point, low aerobic adaptations are driven by duration, not intensity (there are minimums). Some coaches will manipulate low aerobic intensity, for various reasons.


I’ve used the same Friel tests as outlined in Your Best Triathlon and also found his HR zones based off those threshold tests to be too high for both the run and bike. The Coggan zones seem to align better for me.

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Coggan for you, Friel for me. That’s what makes HR so fun :joy:

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I find the boundary between that light resting breathing rate and when the rate increases is a good approx place to be for this. I’d say it falls between 35-25 beats less than my threshold HR. As others say don’t get too hung up on precision. 5 beats here or there is unlikely to make much difference. Just be sure the volume or frequency of this lower intensity work doesn’t impact your higher intensity days. Gradual progression through volume and frequency.


I’ve used HRV Logger and Fatmaxxer apps to use HRV to find my AeT and both repeatedly find exactly the same HR point across multiple tests and rides - for me its 154bpm which is firmly into z3 and typically 85-86% of FTP.

That certainly means I can ride at that power for a pretty long time but its definitely a muscular endurance ride and certainly NOT one I would add to a week planned with other intensity work like a LV plan. For that I would simply ride z2 endurance and either top or middle of the zone depending on other fatigue - thats about 30bpm under my threshold HR as noted above. I really dont think AeT should be thought of as ‘easy’ endurance riding and not a ‘bolt-on’ session but rather a focused target session in your plan. In my current plan an AeT ride IS my intensity session while we are in ‘base’ building phase to max aerobic development.


The paper on DFA-1 has this in the discussion:

Which for me is somewhere in the 85-90% FTP range using the rough estimate of %VO2max based on HRmax and RHR.

Notice the difference in %vo2 for teenage vs adult males.

Like all non-invasive rough estimates, I believe there is a lot of wiggle room.

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Update to the thread after today’s 2hr AE2 workout on the trainer:

100 mins spent in the main set where after considering the advice provided in this thread I tried to keep my HR as close to 135bpm as possible. 135bpm equates to 71% of my HRmax and 79% of my LTHR. For me this seems right around, or just below AeT as it’s still in the “conversational” arena and about an RPE of 3-4.

For context, I also completed a 20 min FTP test yesterday to make sure all zones were as fresh as possible.

For the 100 min main set my NP was 243 and average HR was 135. This equates to an IF of .79, so right in Zone 3.

If memory serves me correctly, this seems very similar to a ride Keegan said on the podcast he does quite frequently where he rides at the top of Zone 2 / low Zone 3 in replacement of LSD rides.