Designing interval training: 10 things you have to know | EP#139

​Designing interval training: 10 things you have to know | EP#139

TTS139 - Designing interval training_ 10 things you have to know

Do you know how to deliberately structure your interval workouts to achieve specific physiological outcomes and maximise the benefits of the hard work you put in? If not, it's time to get started, and this episode will be your guide.

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Discuss this episode!

  • Let's discuss this episode and the topic in general. Post any comments or questions in the comments at the bottom of the shownotes. Join the discussion here!

In this Episode you'll learn about:

  • The importance of time at VO2max and how much of it do you need?
  • Using long intervals to the best effect and optimising their structure. 
  • Using short intervals to the best effect and optimising their structure.
  • How to manipulate interval duration, modality and intensity to achieve a desired workout nature. 
  • How to manipulate recovery interval duration, modality and intensity to achieve a desired workout nature

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Shownotes

Background to this episode 

4:00 -

  • Paul Laursen and Martin Bucheit wrote a research review a few years ago on high intensity interval training.
  • They're now going to turn that into a book. 
  • I'll be discussing these reviews in this podcast as I've learnt a lot through reading them and experimenting. 
  • This episode is only about part 1 of the review - the cardiopulmonary emphasis.
    • How can you get as much air as possible into your lungs. 
    • Then how can you extract as much oxygen as possible from that air into your bloodstream. 
    • Then how can you get as much of that oxygen delivered to your working muscles as possible. 
  • This is the definition of VO2max - it's the maximum volume, uptake and utilisation of oxygen by the muscles.
  • This review also only deals with high intensity intervals that are performed above the anaerobic/lactate threshold. 
    • It's close to, or above your VO2max intensity. 
  • Improving your VO2max is the main goal here.
    • Think of fitness as a continuous thread, from short durations to long durations.
    • For short durations you can hold a high intensity, but for long durations the intensity drops.
    • This is called the power-duration curve, and is very commonly used in cycling (e.g. using the WKO4 software).
    • If you can bring your VO2max intensity up, it pulls on the continuous string and it brings up the sustainable intensity for the longer durations as well as the shorter VO2max-intensities. 
  • For example, say your lactate threshold is roughly 85% of your VO2max, which is a common assumption. 
    • If you increase your VO2max by 3%, your lactate threshold won't necessarily improve by a full 3% but it will still improve. 
  • This is why high-intensity VO2max-intervals are so effective. 
  • They are the tide that raises all boats - focusing on high intensity intervals doesn't mean your endurance at lower intensities won't improve. 
  • In the next few sections I will review some basic concepts that Paul discussed in the interview we did together. 
power duration curve

Power Duration Curve - Click to zoom

Targets of high intensity intervals 

10:15 -

  • Your interval workout can target one, or several of the following:
  • Cardiovascular = VO2max.
    • Increasing your oxygen uptake capability and oxygen utilisation by the muscles. 
  • Metabolic = anaerobic gycolysis.
    • When working at or above VO2max, carbs are anaerobically turned into ATP, which is the basic fuel source of all the cells in the body. 
    • This is a metabolic adaptation because the muscles stores glycogen which are then used to make ATP, without oxygen. 
  • Neuromuscular/musculoskeletal = neuromuscular system. 
    • Your nervous system sends signals to your muscles to contract.
    • You also have musculoskeletal strain. 

Types of intervals.  

12:40 -

  • Long intervals: longer than 1 minute and done at an intensity that is above 90% of your vVO2max or pVO2max (speed or power at VO2max). 
  • Short intervals: shorter than 1 minute usually done at vVO2max or higher. 
  • Repeated sprint training: short sprints with short recovery between them. 
    • Not really relevant for triathletes, often used by team sports players. 
  • Sprint interval training: all out sprints of 30 seconds, with long recovery (2-4 minutes). 
    • Again not as relevant for triathletes, at least not for improving VO2max.

Goals of interval training sessions

13:45 - 

  • The primary goal of an interval session is to spendprolonged time at your VO2max intensity. You want to maximise your T@VO2max.
    • This doesn't mean that your cardiovascular system needs to be at 100% of VO2max to gain adaptatons, but it needs to be above 90% of it's oxygen utilisation and uptake capacity. 
  • You want to do this while controlling for the level of anaerobic engagement.
    • The metabolic adaptations are anaerobic in nature. 
    • You also want to control for neuromuscular load and musculoskeletal strain. 
  • This episode will discuss how to accumulate time at VO2max (T@VO2max) optimally.
    • Although there's no formula for how to do this, we have a lot of indicative studies that show roughly what you can do. 
  • I will use running a lot as an example but this works just as well with cycling. 
  • I will also talk about vVO2max which is your velocity at VO2max.
    • This is your pace when at VO2max. 
  • You need to establish your vVO2max which can be done in several ways:
    • Lab tests, which is good but there are caveats about running on a treadmill compared to outdoors. 
      • It's not always a direct translation. 
      • Also if the stage durations in an incremental lab test are too short, or the step increases on the treadmill speed are too high, this can inflate your vVO2max. 
    • I would recommend using a recent race result or time trial, and plug the values into the McMillanRunning.com calculator. 
      • This will give you your estimated threshold pace and your VO2max pace. 
    • You can also do a five minute time trial.
      • The time to exhaustion at the speed of VO2max is in the 4-8 minute range. 
      • If you do a 5 minute time trial you can use the pace you hold as an estimate of your vVO2max. 
        • This has been shown in studies to closely correlate with lab test results. 

Percentages of intensity

18:20 - 

  • For long intervals (1-6 minutes duration), they will be done at 90-105% of vVO2max. 
  • For short intervals (1 minute or less), they will be done at 95-120% of vVO2max. 
    • You may well have been going into speeds that are 130-140% vVO2max.
  • When you design your workouts:
    • Base them on speed or power, both work well. 
    • Personally, I use speed and/or power on the run, and power on the bike. 
    • Heart rate is not a good measure as the lag is so long so it won't be effective.
  • If you can't use speed or power, use your Rating of Perceived Exertion (RPE).
    • This allows you to self-regulate but it requires good self-regulation. 
    • For example, if your workout was 10x400, you'd try and hit the pace that is the maximum you can sustain for all 10 intervals. 
      • Ideally you want to do your last 2 intervals a little faster than the first.
  • RPE is essential for hill repeats on the run for those not using power, because you cannot rely on pace. 

Example Interval Workouts and T@VO2max

Workout

T@VO2max (>90% VO2max)

T@VO2max (>95% VO2max)

Effectiveness (T@VO2max / total duration)

5 x ( 3' @ 90% vVO2max / 90" @ 0%)

~9 minutes

~43%

5 x ( 5' @ 92% vVO2max / 2.5' @ 46%)

~10 minutes

~38%

3 x ( 2 x ( 2' @ 100% vVO2max / 2' @ 50%))

~8 minutes

~34%

30" @ 105% vVO2max / 30" @ 50% to exhaustion

~6 minutes

~46%

30" @ 105% vVO2max / 30" @ 50% to exhaustion (ref. 2)

~13 minutes

~68%

60" @ 100% vVO2max / 30" @ 50% to exhaustion

~8 minutes

~32%

Refer to Figures 6 and 8 of the review article for source material. 

Tip 1. Short versus long intervals

21:02 - 

  • Short versus long intervals when it comes to attaining time at VO2max has only been studied and compared in a couple of studies: 
    • One example compared long intervals (2 min work/2min rest) to short intervals (30s hard/30s easy - active recovery). 
    • The study showed that there was significantly more time spent at VO2max in the long intervals.
    • However when you change the short interval structure to 60s work/30s rest, then the "time at VO2max effectiveness" was higher than long intervals.  
      • Time at VO2max effectiveness is percentage of time at VO2max as a percentage of the entire interval workout. 
  • These studies suggest that long intervals can be more effective than short intervals to accumulate absolute T@VO2max. 
  • Short intervals with work/rest ratio significantly greater than one can also be very effective, however. In particular when you consider how relatively short the entire workout needs to be to accumulate a lot of time at VO2max.  

Tip 2. Optimising interval duration

23:11 - 

  • Regarding long intervals, the recommendation is to have at least 2 minutes duration.
  • Based on knowledge of VO2 kinetics, we know that when you repeat intervals it takes 1min 20s-2min 20s for you to reach VO2max intensity when working close to vVO2max. 
  • In your first interval, you probably won't reach VO2max max at all unless it's longer than 3 minutes. 
  • There have been experimental studies that have demonstrated this. 
  • They have compared 2min work/2 min rest intervals with 1 min work/1 min rest intervals. 
    • They found the runners reached VO2max in the 2 minute intervals, but didn't at all during the session with 1 min intervals.
    • This study has since been repeated and the same results were found. 
  • They also showed that extending the intervals to 4 or 6 minutes did not improve the peak-VO2 values. 
    • The percentage of VO2max that was reached during 2 minute intervals was 92%. 
    • During 4 min intervals it was 93% and during 6 min intervals it was 92%.
    • This is probably because the intensity was lowered when the intervals got longer. 
  • These studies suggest that 2-3 minute intervals is a sweet spot.
  • Ideally they need to be at least 2 minutes or you likely won't reach VO2max at all, unless you make your rest intervals significantly shorter. 

Tip 3. Rest between intervals

25:57 - 

  • You want to maximise your work capacity for subsequent intervals, while maintaining a VO2 that is elevated from baseline so you can more easily accumulate time at VO2max in subsequent intervals. 
  • It's easy to assume that active recovery would be better because you keep your VO2 at a higher level so it'll be easy to reach VO2max in later intervals. 
  • However, it has also been shown that this can potentially significantly decrease total work capacity.
    • I.e. how many intervals you can do at the required intensity. 
  • It can also trigger the engagement of the anaerobic system during the following intervals. 
  • In the context of long intervals, it is recommended to use passive recovery when the rest intervals are less than 2-3 min duration. 
  • If you chose active recovery, it's recommended to use 3-4 mins at sub-maximal intensity. 
  • Personally I think suggesting 2-3 mins for recovery depends on what the session looks like. 
    • There is a chart in the review article comparing different sessions and time accumulated at VO2max for each. See the table under "Example Interval Workouts and T@VO2max" to review T@VO2max and effectiveness of some example workouts. 
    • The best session in this review is 5x3 min at 90% of vVO2max with 90 second passive recovery.
      • They accumulated almost 10 minutes at VO2max, with an effectiveness of 43%

Tip 4. Total volume of long intervals 

29:09 - 

  • Research is limited in this area. 
  • Based on what we do know, the accumulated high intensity (i.e. more than 90% of vVO2max or pVO2max) during a typical session has been reported as anything from 12-30 minutes. 
  • Example sessions could be 6x2 minutes, 6x600m, 5x3 minutes, 5x800-1000m, 4x4 min, 6x6 min, 4x6 min, or 6x5 min. 
    • This gives you an idea of the total duration. 
  • 6x2 mins (12 min total work) seems low. 
    • In most examples the effective time at VO2max seems to be at most 50% or lower. In other words, with a 6 x 2 minute workout you could expect to accumulate maybe 6 minutes at VO2max (although this of course depends on rest interval duration and intensity). 
  • I would recommend at least 15 minutes total work duration - if you can get 50% effectiveness you'd already have 7.5 minutes at VO2max. 
    • Many workouts produce 0 minutes at VO2max, often unknowingly, so 7.5 minutes would be great. 
  • If you're an advanced athlete, you need to have longer time at intensity, because you need to overload the body to elicit adaptation. 
    • In  other words, you may need to accumulate more time at VO2max to improve it than a beginner athlete. 
    • You may aim for the 25-30 min total work duration workouts to accumulate enough time at intensity. 

Tip 5. Flat vs. hill running for intervals

32:01 - 

  • A study has shown that comparing hill running (6x500m which took 1:40), with a track session (6x600m which took 1:40).
    • Both sessions accumulated some time at VO2max.
    • However the time at VO2max compared to exercise time was clearly higher in the track workout - 44% of workout time spent at VO2max compared to 27% during hill session. 
    • The reason for this is unclear, and the intervals in the hill workouts may not have been long enough to observe the slow VO2max component found in uphill running. 
  • It has made me think that you want to prioritise VO2max sessions to flat running.
  • Unless you're injury prone, in which case hills may be better. 

Tip 6. Manipulate intensity in short intervals 

33:42 - 

  • Now we're getting into short intervals, which are up to 1 minute duration. 
  • You should aim for an intensity that is 100-120% of vVO2max.
    • This is a higher intensity than the long intervals. 
    • However many athletes when they do this workout go way beyond 120%.
    • This often means they can't accumulate enough intervals to spend a lot of time at VO2max. 
  • Several studies have compared increasing intensity of short intervals from 100-120% of VO2max. 
    • They have found that for athletes that are older or less trained, you will want a higher intensity as VO2 kinetics may be slower. 
    • For higher level athletes, you can keep it at closer to 100% rather than 120%, but the number of intervals will be higher. 
  • When you go above 120% of vVO2max in these intervals, studies have shown that the total exercise capacity is impaired.
    • This means the total time at VO2max is typically lower than going 120% or below. 
  • The review authors suggest if you split your high intensity workouts into several sets of these intervals it may be effective. 
  • As a general recommendation, I would steer you to stay in 100-120% vVO2max range. 

Tip 7. Duration of short intervals

36:17 - 

  • There are a few things here that we don't know because they haven't been studied.
    • E.g. How does a 30/30 workout (30s hard, 30s active recovery) compare to a similar total duration workout using 60s work/60s active recovery. 
  • What we do know, is when you increase the work duration to rest ratio, going above the 1:1 ratio, the time at VO2max increases significantly.
    • For example, one study investigated in well trained triathletes 30s work/30s active recovery, compared to 60s work/30s active recovery.
    • The total time at VO2max was 9 minutes in the 60/30 workout, and 1.5 minutes in the 30/30 workout. 
    • This shows how important increasing the work to rest ratio in short intervals can be. 
  • Older and less well-trained athletes that have slower VO2 kinetics are recommended to use longer work intervals (e.g. 60/60 instead of 30/30). 
  • More well trained athletes may need to adapt the total amount of work that they do. 
  • For example: a less well trained athlete might do 15 x 60s hard/60s easy, which gives a total of 15 minutes at a hard intensity. 
  • A well trained athlete might do 50 x 30s hard/30s easy, which gives a total of 25 minutes at a hard intensity. 
    • This is why you need to know your vVO2max and adjust your intensity accordingly to ensure you can maintain the intensity for all intervals. 

Tip 8. Rest interval intensity in short intervals

39:07

  • A study has shown that during a 30 seconds on/30 seconds off workout, when comparing recovery interval intensities of 50% vVO2max and 67% vVO2max to a baseline of passive recovery, "small" and "very large" time at VO2max improvements were seen, respectively. These same improvements were seen in the workout effectiveness.
    • However, increasing the recovery intensity further to 84% moderately reduced T@VO2max.
  • It is recommended to keep recovery interval intensity at ~70% vVO2max.
  • If your intervals are of very high intensity (e.g. 120% vVO2max) you may need to scale back the recovery intensity a little bit. 

Tip 9. Sets in interval workouts

41:12 - 

  • Using different sets in interval workouts is common practice when designing sessions. 
    • By sets, I mean for example doing 3x3 mins at VO2max intensity, then doing an active rest for 5 minutes, then doing another 3x3 minutes. I.e. 2 x ( 3 x 3' / 3').
    • In total you do 6x3 mins but you have an additional active rest in between intervals 3 and 4.
  • This has been consistently shown to reduce the total time at VO2max.
  • However, there is a time and place for splitting workouts into sets.
    • For example, if you want to do a 30 sec on/30 sec off workout and you want to a total of 30 minutes for your main set (i.e. 30 repetitions). 
    • If you know this is likely to be challenging because of your fitness level, you may aim for 36 minutes but split it into 3 different sets - 3x12 minutes.
    • This would be 12 times 30/30, then 1-4 minutes active recovery, then another 12 times 30/30 etc. 
    • You want to achieve 36 minutes because you have to take into account that when you start a new set it'll take time for your VO2 kinetics to rev up again. 
    • If you don't increase the main work time, you will lose time at VO2max.

Tip 10. Warm-up 

43:46 - 

  • There should be a very short, or non-existant delay between your warmup and the start of your main work. 
    • Otherwise your time needed to reach VO2max will be longer. 
  • Warmup intensity can be around 60-70% of your vVO2max or power at VO2max.
  • Finish with some strides/builds/skipping ropes at the end of the warmup. 
  • The warmup is important in this context. Just remember not to take a long break between it and starting the work. 

Key takeaways

  • Today we've only discussed time at VO2max, but the second part of the review discusses neuromuscular/musculoskeletal train and load, and metabolic demands. 
    • For example, very long intervals (e.g. 5x4 minutes) puts a high demand on your anaerobic system. 
    • This is because your anaerobic energy that you produce will be higher at the end of the intervals. 
  • There is no one magic workout, but there are some that work better than others.
    • See the section "Example Interval Workouts and T@VO2max" above for some examples.
    • The adaptations that you will elicit depend heavily on how well you design your interval workouts. 
    • Manipulating variables such as interval intensity, duration and rest can dramatically change the workout outcome. 
      • Particularly how much time you spend at VO2max. 
  • For long intervals, if you have equal work/rest ratios you want to go 2 minutes or longer in duration. 
    • Ideally not including sets, and for enough repetitions to be beneficial. 
    • For example, 8x2 mins on/2 mins off, or 6x3 mins on/3 mins off. 
    • Keep yourself at 90-100% of vVO2max. 
      • For 2 min intervals, 95-100%.
      • For 3 min intervals, 90-95%. 
  • For shorter intervals, you want the work/rest ratio to be significantly larger than 1 (1.5 ideally). 
    • For example, 60sec on/30 sec off. 
    • In these short intervals, keep in mind that intensity should be high but not too high. 
    • It's easy to go too high in these but it's important to accumulate total work duration.
    • Keep them to 100-120% of vVO2max. 
    • You want an active recovery (e.g. jog at 70% VO2max) to keep VO2 elevated from baseline. 
  • Your main goal is not to run the intervals as fast as possible, but start considering 'can I add another interval at the same intensity'. 
    • This will cause more adaptations. 
  • Knowing your fitness level is crucial.
    • You have to have your own benchmarks to base these intensities off of. 
    • This can be time trials, races, lactate tests etc. 
    • For running, go to McMillan running and use their VO2max calculator.
    • For cycling, do a 5 minute time trial, or an FTP test. 
      • You can multiply your FTP by 1.176 and that gives your power at VO2max, assuming your FTP is roughly 85% of power at VO2max. 
      • It won't be perfect but it's a good starting point. 
  • Don't get caught in paralysis by analysis by all of this!
    • You can use the examples from this episode for effective workouts e.g. 60sec on/30sec off at 110% vVO2max, or 6x3 mins at 90-95% vVO2max. 
  • All of these tips can be used as baselines and think about what will happen to your time at VO2max when you manipulate these different variables. 

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    Connect with host Mikael Eriksson

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    Hi! I'm your host Mikael,

    I am a full-time triathlon coach and an ambitious age-group triathlete. My goal is podium at the Finnish national championships within the next few years.

    I first started the website Scientific Triathlon in autumn 2015 as a passion project to share my learnings with a larger triathlon audience. Later on, in early 2017 I started the podcast That Triathlon Show. 

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  • Brad says:

    Awesome podcast man. I love this type of dive into training. Very specific and actionable information. Maybe some sprint interval training for race specificity when chasing down a muggle, especially on the bike or the end of the run? I tend to incorporate these into my rides, and a fairly comparable set of strides into my runs.

    • Thank you Brad! Yes, some sprints definitely can be a good addition to any plan. I tend to give some strides to my athletes once per week at the end of an endurance ride, and also often some sprints on the bike as part of an endurance ride. The important thing is to take enough recovery between each sprint.

      Of course, the purpose of sprints and strides is different than this particular episode’s scope. Developing maximum power/speed and running technique in the case of strides, versus developing aerobic capacity (VO2max) centrally and peripherally in the case of the interval workouts described in this episode.

  • Rickard says:

    These are my favorite kind of episodes, when you break it down and lay it out here for us. Thank’s Mikael.
    One question; Since I take it that the VO2max intervalls are very efficient – When would you wan’t to do he zone 3 and 4 work? Is this only for race specificity at the build up for a race or do you still throw in, for example a zone 4 intervall session, every now and then? How to think when mixing it up?

    • Glad you like these episodes Rickard! I also enjoy producing them a lot, but they are very time consuming, so I can’t do them all the time.

      To answer your question – it depends. It’s not just for race-specific training in my opinion. For example, I’m a fan of building fitness on the bike in the off-season by doing at least one sweet spot/threshold workout and one VO2max workout.

      I’m also a fan of easing into more intense run training (e.g. when starting training again after a planned recovery period, or when reintroducing intensity after an injury) by doing Z3 intervals and continuous tempo runs.

      At times, it can be useful to focus more on VO2max. E.g. 2 VO2max intervals per week on the bike and no other hard bikes. This can massively boost fitness. But most of the gains from this type of training will be reaped after 6 weeks or so, so after that, you would want to revisit some other type of training, e.g. sweet spot training.

      Hope this gives you some ideas of how to think about it!

      Mikael

  • Cruise says:

    Hello Mikael!

    This is very interesting. I thought you were an advocate of Seiler’s 80/20 ideas. This doesn’t seem to fit. For example, his research doesn’t find a difference in effectiveness of VO2 intervals vs. 4×8′ in his zone 3. Your off-season bike sweetspot work and your tempo runs you mention, as well as your last comment of revisiting sweet spot training after about 6 weeks of VO2max…

    All of this is contrary to Seiler’s 80/20 research…especially the sweet spot and tempo training, as that’s right in the middle of the “no go zone”

    Can you please reconcile this for me? Or have you abandoned polarized training?

    • Good questions.

      I am not, and have never been of the opinion that all interval training should be done above the anaerobic threshold as per the polarised training approach. I do believe in, and mostly follow and advocate keeping 80% of your training easy, although I don’t think it’s gospel, especially for age-groupers with limited time to train. Sometimes, blocks with more intensity, e.g. a significant amount of sweet spot training, can be gret to build a strong base and ssee relatively large fitness improvements on a limited time to train, as long as recovery periods are also used in the big picture planning.

      More generally about Seiler’s research, the 80/20 approach doesn’t at all place preference on any particular types of intervals, as long as 80% of the training is easy (below the aerobic threshold). This is well-established. Polarised training, where most of the 20% is done as high-intensity work is quite well established, but it’s worth noting that when looking at the intensities used by the athletes in the studies, they do include mid-range intensities as well, often in an 80/10/10 distribution (which is still emphasising high-intensity much more than the norm). The more polarised concept has some solid research behind it, but it’s not as well-established as just the fact that 80% of training should be easy, and there’s almost no data from triathletes.

      The study comparing types of intervals that you are referencing is a separate research study and really has nothing to do with 80/20 or polarised training in itself. And the findings are not yet well-established, the results haven’t been replicated anywhere, so it’s really just one small study. Although very interesting and well-designed of course!

      The key for me is that triathlon, from the Olympic distance and up, is a lot about muscular endurance, and you can’t not do those sweet spot rides and tempo workouts if you want to build muscular endurance. I see in the athletes that I coach that this is the case. They can get away with not quite managing to get in the polarised training phase that most of my athletes (intermediate and advanced) do and still have solid races. But they can’t get away with not getting in a solid period of muscular endurance training and still perform as expected or better. In particular for 70.3 and Ironman.

      So to some up:
      -For certain training phases I use a completely polarised (high-intensity and low-intensity) approach

      -For most phases I use an 80/20 approach, but the content of the 20 depends on the athlete and the goal race. Sometimes it’s more polarised, sometimes less.

      -In the specific preparation phase for 70.3 and Ironman, the 20 is definitely mostly between the aerobic and anaerobic thresholds, i.e. NOT polarised, and this is what to me seems to be a far superior method.

      -In some periods, depending on the athlete and the time they have available (more so for very time-crunched athletes) I completely forego 80/20 as the limited time they have available doesn’t allow for sufficient stimulus unless they have more of their training be of slightly higher intensity, e.g. tempo/sweet spot.

      Hope this answers your question!

      • Cruise says:

        Thank you, Mikael, for the great explanation. That makes perfect sense. I recently heard an interview with Dr Seiler and he spent some time discussing these exact topics (interval lengths, zones to target, some age group studies that confirm his research, etc) I was just a little confused because I thought that you more closely followed his findings. You’re actually following a much more typical (for today) training regimen than I thought; more sweet spot based. It is not consistent with Seiler’s findings or his current recommendations. Thank you for the clarification.

        Forgive me, I do have to offer some clarification on a couple of points:

        1) The preference of intervals is certainly defined by his research. They are found within his Zone 3, not Zone 2 (“sweet spot”). This is not dependent upon available training time or sport or race distance. He has recently addressed this argument in his presentations.

        2) The 4×8′ intervals that I mentioned actually are directly from his work (Seiler S, Jøranson K, Olesen BV, HetlelidKJ. Adaptations To Aerobic Interval Training: Interactive Effects Of Exercise
        Intensity And Total Work Duration. Scand. J. Med Sci Sports 23, 74-83, 2013). If I recall correctly, that study was a group of recreational cyclists who averaged 6 hours of training per week (ie “time-crunched”). The 4×8′ interval set proved to be most effective in the study. It is worth noting for two reasons: a) it speaks directly to those who claim that polarized training isn’t effective for recreational time-crunched athletes, and b) it was a time-to-exhaustion study where the 4×8′ intervals dramatically improved TTE over other interval schemes — that is, the “muscular endurance” that you referenced was the key improvement here. I would be interested to look into the specifics and see if the longer interval groups dropped into “sweet spot” due to interval length, suggesting an advantage of higher intensity intervals over sweet spot intervals, as Seiler advocates.

        3) As to training distribution, it is true that the 20% distribution varies. This variation, however, seems to be related the sport being studied. Seiler has addressed these differences. For example, he compares a rower who can drop into Zone 3 and hold it to a runner or cyclist who has to contend with hills, causing them to drop out of Zone 3 or exceed the planned effort. The more variation the environment brings to the sport, the more likely there will be incursion into Zone 2 from both sides. He also attributed some of the Zone 2 time logged as transitory because his studies include total workout time and total time spent in each zone. There are always going to be transitory periods as one moves from Zone 1 to Zone 3 and back again during an interval session as well as Zone 1 training that creeps into Zone 2 due to exceeding planned effort or external factors, such as heat, stress, etc. That is not to say that he doesn’t see a purpose to some selective Zone 2 training under specific circumstances, but he does not agree with conflating “sweet spot” training and his suggestion of “polarized training.”

        Thank you again, Mikael, for your explanations. It is tremendously helpful. And thank you for your great work in your podcasts and articles. They are among the best!

        • You’re right, I don’t follow a polarised training approach over the whole season. I do follow it in certain parts of the periodisation (when that is depends on the athlete and their goals).

          1) It’s not that clear-cut. Look at Seiler’s presentation here for example. Table 2 from the Sandback study. World-class national skiers from that study do 50% more moderate intensity training than high-intensity training. The point of his argumentation is not that we need to completely move away from moderate intensity training, but put a higher emphasis on high-intensity training. They are not mutually exclusive objectives.

          Also, look at the TID of Bente Skari (the “world-class skier”) in the presentation. A lot of Z4 training going on there, and that is something that depending on how you define it can be classed either as Seiler’s Z3 or Z2. For example, 95-100% of FTP would technically be Z2, but physiologically, we’re close enough to the anaerobic threshold that it really isn’t any different from doing intervals at 100-102% FTP, except for technicalities.

          And the muscular endurance intervals I prescribe are not just sweet spot, I classify threshold there as well to some extent. I don’t really care about “labeling” my training prescriptions, but let’s say your intense workouts of the week are a threshold workout and a VO2max workout. That might look like 80-10-10 or 80-0-20 depending on how you look at it. And of course, going back to the Bente Skari example, her career TID would probably be something like 80-15-5 when you consider that two thirds of Z4 in a 5-zone system fall below the threshold.

          2) It is a fantastic study, and I can’t wait to see it replicated. But again, those 4 x 8′ intervals probably fall right around thresold. I really don’t think it makes a difference how you label them – Seiler Z2 or Z3, but the fact is, for many of these athletes they were probably done at 95-100% FTP, not at 100+ % FTP.

          The study does show that over this study duration, it is more effective to use these kinds of intervals, and I do agree with that. If I have 8 weeks to get an athlete in peak shape for a race that isn’t an Ironman, I will default to threshold and VO2max intervals, and not sweet spot. But as we’ve seen from the examples (e.g. world class skiers referenced above), you can’t do that all year long and keep improving. You need to change up the stimulus sometimes. You can’t take an 8-week or 12-week or 16-week study and expect it to work for years and years with similar results. Or maybe you can, if the alternative is doing ONLY sweet spot training for years and years, but that’s not the point.

          And also keep in mind, it’s still just one study and it needs to be replicated before we can claim it is any sort of established fact.

          3) Yes that’s a fair point. No arguments there.

          I would be interested in reading/watching/listening to some of the resources you’ve gained your perspective from. Would you mind linking them?

          Thanks a lot,
          Mikael

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