Fast Talk Episode 168: How Much High Intensity Training Do We Need?

In the latest episode I believe @trevor talked about how he considered 100-110% to be “threshold” level training rather than VO2max.

I thought that 100-110% FTP is the range that power falls into when doing @stephen.seiler’s classic 4x8min session, and this session is about raising aerobic capacity / VO2max.

So have I got confused or mis-heard? Or are they just explaining the same thing in different ways?


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In the lab, we do not use FTP, or at least I have not in previous studies. Here is a table from a big 2017 study where 63 cyclists did multiple sessions of the 4x4, 4x8, and 4x16min HIT prescriptions. We used power during a 40min Time Trial as a calibration…


@stephen.seiler, I’m curious on your opinion regarding high intensity structure. The coaches at Fast Talk seem to have a preference for structured high intensity work focusing on targeting specific energy systems (“Threshold work” versus “VO2 work” versus sprints). I got the sense from your comments on older podcasts (episode 75) that there was such an overlap between systems, that any time above a certain effort level (~86% HR max) will lead to similar adaptations and the amount of time accumulated in this “red” zone was key. Hence the similar results in the 4X16min, 4X8min, and 4X4min groups. I think it’s a very relevant question to help one determine do they do their high intensity work with specific interval times/targets or something with less structure (zwift race, strava segment challenges, group rides, informal races, etc). Thanks.

Thanks very much @stephen.seiler - I never heard you specify % of the FTP, but others have attempted to estimate what range it would fall in. That all makes sense now.

If you’ll indulge me another question: I believe you talk about “time in zone” being the important thing, and measuring this as how much time you accumulate at 90-95% HRmax - is that right?

My heart rate is quite slow to respond so, for example, doing a session of 4 x 6min efforts (working my way up to 4 x 8min) I only accumulate half that time at 90-95% HRmax. See attached image where I have highlighted the time spent in that zone:

  • 1st rep of 6min = 0 time in HR zone
  • 2nd rep = 3min time in target HR zone
  • 3rd rep = 4min time in target HR zone
  • 4th rep = 5min time in target HR zone

All 4 reps were done at consistent power (109-110% FTP) so I did 24min of work, but only achieved 12min in the target HR zone. I guess without going into a lab we’ll never know what’s happening inside the body, but does this slow responding heart rate suggest any less benefit in the session to someone who’s heart rate responds faster?

Thanks very much for your input

We dug into the weeds on this question in a big study published about 4 years ago. Here is the link (free access):


Actually, I have argued for using “Tolerable Accumulated Duration” as a guide for both prescription and execution of interval sessions. So, within reason, you can mix and match interval prescriptions. There is no magic formula. If I prescribe 4 bouts of 8min work duration with 2min active recovery or 8 bouts of 4min work duration with 2min active recovery, there IS a difference in “work density”, but the overall physiological and perceptual responses end up being basically the same. So, you have both intensity and total duration as variables in the equation that you can manipulate and adjust. Add repeats at the same intensity before you up the intensity, as a general rule. And, be flexible, give your athletes SOME leeway in how they want to accumulate the work within these basic parameters. Then they “own” the workout and that is more motivating for many.


As to the HR time in zone issue, I do not get too caught up in that. I do think there is variation in HR kinetics and also in the stroke volume-HR relationship across athletes. This is related to age, training, genetics, etc. It can also be a fatigue issue. It is quite common for athletes to experience that “the brakes are on” and HR is too low for workload. If this is unusual for you, take it as a sign of lingering fatigue and do not double down on intensity!

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Thank you @stephen.seiler that is truly excellent information and advice. We are lucky to have you available like this.


I have a question regarding ‘power at VO2max’. In this episode it sounded like this was considered a specific power output, similar to MAP (Max Aerobic Power.) This terminology seems widespread, and not limited to this podcast.
My understanding of this area is due to the slow component, many different power outputs above FTP will get you to VO2max. It seems to me that ‘power at VO2max’ is not a good name for what is being described. I’m not sure what is meant by this, as what power is measured when testing for VO2max will depend on the test protocol used, so even if we take this value from a VO2max test it seems rather imprecise.

So, what’s the deal with ‘power at VO2max’? Should I just translate this to MAP in my head?

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I agree with you that power at VO2max (or VO2 peak) cannot be viewed as a fixed value. This is due to day to day variation, fatigue, etc. An too abrupt increase in load can lead to mismatches as well, but if load increase goes too slow… So, there are issues, but we do the best we can to generate a reference point for “maximal values”: HR, Power, Oxygen consumption. They align pretty well most of the time. Here is a nice graphic from a good review paper. I annotated it a bit.

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Great conversation! @gerrard thanks for getting it started.

@stephen.seiler has done a great job explaining the various nuances. Fully agree with everything he said. And truly appreciate that he’d take the time to respond on our forum.

I do feel I should pipe in just to clarify a few messages that @chris and I have been trying to communicate on the show because I feel we still have some work to do on that…

First and foremost, a really important message that I hope we do eventually communicate is that training is not about a specific power or heart rate number. Going from 300 watts to 305 watts does not suddenly change the energy system you are targeting. It’s a broad range and more importantly a range that changes day to day.

We actually included that bit with Kristin Armstrong in the episode intentionally to make that exact point. We’ve heard from many listeners who believe they are only training their threshold power/aerobic system if they are sitting right at FTP (which, as Dr Seiler pointed out, isn’t truly a physiological number itself.) I liked having a world champion time trialist say “nope, you’re still training it at 110%,” again emphasizing it’s a range.

To address your question, yes, I am big on targeting energy systems, but the intensity of the workout is only one of many factors and often not the most important factor. When I prescribe workouts to my athletes I give them a very large heart rate/power range or sometimes my prescription is a simple “as hard as you can go for the timeframe.”

I actually talked about that on a recent episode where I was doing hill repeats and one week I did them at 350 watts. The next week my legs weren’t as good so I did them at 320 watts. But I was still hitting the same energy system and accomplishing the same purpose. But as a percentage of FTP the one week was about 110% and the other week was about 97%.

What’s more important is the execution. I actually just wrote an article addressing this question. If you’re interested you can check it out here: Your Guide to High-Intensity Training - Fast Talk Laboratories.

In the article I use the hill repeats as an example. I’ve seen people do them where every repeat they are letting their power drop 20-30 watts. To me that’s poor execution, not because of any particular percentage but because they are never truly stressing their aerobic system and may not get an adaptation out of the workout. To get the adaptation, I feel the workout needs to be done at a consistent wattage. I’m not as concerned about exactly what that wattage is as long as it’s in the right range. That’s an example of what I mean by “execution”

In that same article, I addressed VO2max intervals. I do think the name is a misnomer. I don’t think they truly train your VO2max. And actually on the episode we have a great segment with Dr Seiler saying that VO2max quickly becomes untrainable. LIkewise, in a previous episode we had a fun conversation with Sebastian Weber about VO2max intervals and how both of us felt they are confusing interval with no clear purpose.

So just to make sure I haven’t confused anyone, in the episode we were not trying to promote VO2max intervals. We discussed them because they are popular, but I don’t personally prescribe them and as someone who really cares about targeting energy systems, I find they can be all over the map.

Great conversation! Hope I clarified a few things about he episode.



Thanks @trevor I do hear you about the range, and have saved the podcast to my favourites for future listening. Along with episode 101 “Zones are a range”, and episode 7 “Do we need training zones”. I’ll come back to all of those when I need the reminder not to get too precise or stuck in my thinking.

HI @gerrard,

Glad that helped and thanks again for the question! I know this is a really tough topic and we get feedback all the time that it sounds like we’re contradicting ourselves. It is a really nuanced discussion to say that specific numbers don’t matter, but then to say that execution is critical, you need to target a range and it’s important to be consistent in your intervals.

But I’ll also say that I personally think one of the biggest things that separates elite endurance athletes is the years they spent learning those seemingly contradictory nuances. Effective execution is actually a really hard thing to do!


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I’m curious what you’re prescribing then with regard to high-intensity sessions if you need to work on an athletes max aerobic power.

I personally love hitting 4-6 min intervals around 90-95% of MAP (usually is 115-120% of FTP). I find that they are extremely relatable to a lot of race scenarios (establishing a break, a race-winning move, many durations of climbs in the Midwest) and pay huge dividends.

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Lots of great comments on here already.

I just wanted to add this chart, used this for many years. It is based on % of MAP and you can see that the interval intensity and duration you have found success with falls in line with the chart. Or close, the chart says 4 intervals, 3-4m intervals.

You can find info and studies on this if you just google Guy Thibault Interval Training Model.

I worked with him for a number of years and have used this model successfully.

There are many who have over-trained athletes with the same shart, often by too many sessions per week…something we have heard over and over.


@stephen.seiler @trevor
Congratulations on another excellent episode with lots of useful insights. There is still one question which sits heavy on my mind about ‘Polarised Training’ which I think is more easily explained with an example. It is perhaps not quite the right thread but it came up again in this episode so seems as good a place as any. I thank you for your indulgance.

A rider trains 8-9 hours per week, consisting of short intervals in the week and longer rides on a weekend, something like this:
Mon (Rest)
Tue Hi-intensity intervals (1hr)
Wed Mid-intensity (1hr)
Thu Short aerobic ride (1hr)
Fri (Rest)
Sat Tempo (2hrs)
Sun Endurance (4hrs)

The purpose of the Saturday ride is to build Muscular Endurance (as described by British Cycling), whilst the Sunday ride is to build Aerobic Endurance. I presume this contradicts the polarised model due to the relatively high volume of Tempo on Saturday, so here is my question:

If we drop the intensity of the Saturday ride from Tempo to Endurance of course there will be less fatigue, but what about the adaptions? Do you believe reducing the intensity is preferred because there will be no less adaption with 6hrs of endurance over the weekend rather than a mix of Tempo and Endurance (or Green and Yellow zone)?

This is a great example of the need to not get too rigid on this “80:20 ratio” of aerobic, low-stress sessions and high intensity/high-stress sessions. I have similar amounts of high-intensity work (one short hard and one longish hard session per week). However, my rides mid week are a bit longer and I only take one rest day such that I end up with maybe 10h of riding and 1h strength per week. SO, I find myself a bit more “on the edge” of what I tolerate at my age, even though my ratios look “better”. You are taking two rest days and that is certainly going to help you handle the high intensity load. If you are feeling good and able to mobilize for those hard days (heart rate responsiveness is something I look at closely), then I would say you have a sustainable balance. You can always experiment a bit to see how you handle a bit more volume mid week, or a little less tempo work. I think that when you are in the basic ballpark on easy and hard day balance, then it is more about individualization and adjusting to extraneous life stress from week to week. I think fiber type also plays a role here. I tolerate short really high-intensity sessions better from a recovery standpoint than I tolerate the 2-3h rides with lots of “threshold zone work”. If I go fully glycogen depleted, then I will be physically much more fatigued in subsequent days. I know I have a generous FT fiber portion and that is why I think this is the case. So, I have to account for this in my training and recovery time-course. This is just me, n=1. However, I think this is the recipe. Learn from the wisdom of the successful endurance crowd, but then tweak to your own realities. Use the polarized approach as guard rails and then adjust a little up or down based on your training status, your individual physiology, and how it all fits in with the rest of your life from week to week.


I hope that was helpful.

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Definitely! Thank you.

@stephen.seiler - Hilsen fra Asker-området! Love the FastTalk site and podcasts.

You mentioned about 30’ mark in podcast that a lot of people keep pounding HIIT/vo2max with little to no (or even negative) effect, unable to tolerate the intensity or already being fairly well developed, near vo2max.

i was being curious about 4x8’ and 8x4’ in z3 (red) vs. 4x15, 3x20’ in z2 (amber) in regards to mitochondrial development and improve efficiency (arbeidsøkonomi)? listening to Kristin Armstrong doing 20’ at 110% sounds brutal but heck - she’s a champ and more/larger mitochondria than all of us in this thread combined.

would grown-ups like us in the 50+ segment, training 6 days per week, 10-13hrs benefit from 2x 4x8’ or better off with 1x 4x8’ and 1x 4x15/3x20’ intervals if the overall goal is 120-150km endurance rides??

Enjoy the summer down south in Kr.sand.


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