Comparing Metabolic Response to Hard-Start VO2max Intervals

This week I want to look at VO2 and SmO2 response to one of my favourite VO2max workout protocol. We will see how systemic metabolism responds to hard-start VO2max intervals. The experiments were fascinating and I’ll try to give some recommendations to dial in your own individual workout prescription.

Hard-Start Intervals

The principle of the hard-start VO2max interval is to set off at a very high intensity in order to rapidly reach near VO2max (for which 90% HRmax is a good target for most people). Power can then be slowly decreased so that HR (and VO2) remain near-maximal, but the workload can be sustained for 5+ minutes without blowing up.

More specific directions:

  • Set off at extreme-intensity, ie. at a power above your VO2max workload (>PVO2max). This should feel near-maximal effort.
  • This should elicit 90% VO2max within ~90 seconds for most people.
  • 90% HRmax can be used as a target for most people, but HR is not a great corollary to real measured VO2.
  • Once above 90% VO2max or HRmax, begin to decrease the workload so that you remain above that 90% target, but you can sustain the full prescribed duration (6+ minute intervals at >90% VO2max should be possible).
  • Work intervals can be maintained longer, because the decreasing workload accounts for VO2 slow component and decline in gross efficiency due to fatigue. So more time can be accumulated >90% VO2max.
  • Recovery intervals can be longer than the typical 1:1 work/rest, because the hard-starts are more effective at rapidly stimulating aerobic metabolism. So we aren’t as concerned with letting VO2 drop between work intervals. This should allow longer work intervals and more repetitions, again allowing greater accumulation of time at >90% VO2max.

Systemic & Local Oxygenation Refresher

Total haemoglobin (tHb) and Muscle Oxygen Saturation (SmO2) reported by the Moxy monitors tell us the total blood oxygen carrying capacity or blood volume (tHb) within the local tissue and the percent saturation of that capacity (SmO2 in %). The absolute and relative values begin to build a picture of local muscle aerobic capacity, ie. how well the muscle can extract and utilize the available O2 to produce power.

Volume of Oxygen Consumption (VO2) should be well familiar by now, but read here for a refresher. The VO2 Master Pro analyses how much oxygen is getting in, and how much is coming back out. This reflects net systemic O2 consumption and can be used to determine how hard the body is working relative to maximal oxygen uptake (VO2max).

With that reminder of SmO2 & VO2, let’s jump right into the metabolic data for a 3x6min VO2max workout that perfectly demonstrates the benefit (and the challenge) of a hard-start interval.

3x6min Hard-Start VO2max Intervals

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3x6min Hard-Start VO2max – Muscle oxygenation (Moxy) & VO2 Master Pro data

Top chart shows:

  • Blue line is Muscle Oxygen Saturation (SmO2 in %) for Right quadriceps
  • Pink line is SmO2 for Left quad
  • Dotted pink & blue lines show Total Hemoglobin (tHb in g/dL) for each leg respectively (we won’t focus on this yet)

Bottom chart:

  • Yellow line is Power
  • Red is HR, highlighted above 90% HRmax
  • Blue area is Oxygen Consumption (VO2 in mL/min), highlighted above 90% VO2max
  • Note: unfortunately the VO2mask ran out of batteries before the third interval!
  • Vertical dashed lines bound the first 90sec of each 6min interval
  • Arrows indicate the hard-starts

On the first interval the athlete tried a half-hearted hard-start (indicated by the arrows). Power wasn’t high enough to stimulate the aerobic system as quickly as it could have been. As a result HR & VO2 only rose to ~80% of max in the first 90 seconds.

The athlete only reached 90% HRmax and VO2max near the end of the 6min interval (highlighted along the respective HR & VO2 lines in the bottom chart). Muscle oxygenation (SmO2 in the top chart) also gradually trended downward during the interval to a minimum at the very end of the interval.

For the second interval I took control of the athlete’s smart trainer and raised the hard-start power target significantly! This time the interval started at 355 W instead of 325 W. The athlete reached 90% HRmax & 90% VO2max rapidly, and de-saturated SmO2 to a minimum within the first 90sec. I manually adjusted resistance down as the athlete approached the target, and all metabolic values stabilized around their target values.

Average power after the hard start was the same in the first two intervals. The only difference was a harder-start on rep #2, and the athlete was able to accumulate more time >90% HRmax (1:23 vs 5:10) more time >90% VO2max (1:44 vs 2:20) and more time at minimum %SmO2 (although I don’t have exact targets for SmO2 yet).

Average VO2 for the 6min intervals was 82% of VO2max in rep #1 compared to 86% in rep #2. Although this could still have been higher to make these intervals even more productive.

Finally, the third interval.. unfortunately the VO2 mask ran out of battery so we couldn’t collect respiratory data. But if we look at the trends in Power, HR, SmO2, and VO2 I would guess the quality of this interval was somewhere between the first and the second intervals:

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3x6min Hard-Start VO2max – Visualizing trends

Same chart as above with:

  • Pink arrows indicating SmO2 (%) trends
  • Red arrows indicating HR (bpm) trends
  • Blue arrows indicating VO2 (mL/min) trends

Notice the third hard-start was subdued like the first interval, and again HR shows a slower rise toward 90% HRmax, instead of the rapid rise and plateau observed in the second interval.

Likewise SmO2 shows a similar trend to the first interval, with gradual de-saturation to a minimum, rather than rapid de-saturation and plateau at that minimum. I suspect we would have seen a similar trend in VO2, with a slower rise toward 90% VO2max and accumulated time >90% VO2max somewhere between that of reps #1 and #2.


  • This workout was able to demonstrate the difference between a hard-start and a… hedged-start strategy in stimulating the aerobic system up to maximal, and accumulating time at our target metabolic states of >90% HRmax,  >90% VO2max, or minimum %SmO2.
  • Hard-starts must be hard, with intensity well above what could be sustained for the entire interval duration in order to maximize metabolic training stress of the interval.
  • We also demonstrated that for this athlete, 90% HRmax seemed to slightly over-predict 90% VO2max. As in at 90% HRmax, the athlete was slightly under 90% VO2max. They were able to sustain >90% HRmax for over 5min during the second interval while hovering just about at 90% VO2max, but strictly speaking only accumulated 2.5min at or above that target.

Hard-start and weak finish

The athlete above showed the typical hard-start VO2max interval, where after the first 90 seconds the interval workload more or less stabilized at a steady power output. But of course I have to share what my own intervals look like when blood flow restriction begins to affect my Left leg.

Spoiler: systemic metabolism remains elevated, but power output decreases dramatically.

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3x6min Hard-Start VO2max – Muscle oxygenation (Moxy) & VO2 Master Pro data

Top chart shows:

  • Blue line is Smo2 (%) for Right quad
  • Pink line is SmO2 for Left quad
  • We’re still ignoring hemoglobin, but it’s there if you know what you’re looking at

Bottom chart:

  • Yellow line is Power measured by dual sided crank-based 4iiii power meter
  • Blue line shows Right leg Power (good leg)
  • Pink line shows Left leg Power (bad leg)
  • Red is HR, highlighted above 90% HRmax
  • Blue area is Oxygen Consumption (VO2 in mL/min), highlighted above 90% VO2max
  • Vertical dashed lines bound the first 90sec of each 6min interval
  • Arrows indicate the hard-starts

No hedged-start for me! I have experience with exactly what intensity works for me, so I jump straight into a very hard-start. Again the first 90sec is crucial, and I am able to get HR up nearly to 90% HRmax. More importantly, VO2 reaches 90% VO2max and remains elevated for the entire remaining interval duration. Looking at muscle oxygenation, SmO2 still shows a more gradual decline through the first interval.

All three 6min intervals are very consistent in time accumulated above 90% HRmax and 90% VO2max, and for SmO2 de-saturation. One interesting comparison to the other athlete above, is that for me 90% HRmax seemed to slightly under-predict 90% VO2max. Total time accumulated >90% HRmax was 11:29, compared to 14:44 above >90% VO2max.

However, instead of power stabilizing after 90sec, my Left leg (pink power line in the bottom chart) continues to decline as blood flow and oxygen delivery becomes restricted. Meanwhile my Right leg (blue) takes over and maintains a more stable workload. The net result is that overall power output (yellow line) decreases steadily through the entire interval. My power output through the hard-start interval declines by over 25%, compared to the athlete above who stabilized with only a 15% decline.


  • Hard-starts must be hard! Attaining anything near VO2max requires a near-maximal effort. Worth repeating.
  • Two similar workouts by two different athletes show the difference in metabolic response, especially in how well HR correlates to VO2. I would still suggest 90% HRmax can be used to estimate 90% VO2max, but your individual response will vary.
  • If you are training at home with power & HR, HR should rapidly reach a peak in the first 90sec of the interval (some people may be faster) then plateau for the remainder of the interval. Power may also plateau or continue to decline after that initial 90sec.
  • I wonder how long these VO2max intervals could be maintained before either exhaustion, or VO2 cannot be sustained above 90%? Billat et al, 2013 found subjects were able to spend over 20 minutes continuously above 90%VO2max! So there may be room to push these 6min intervals further.
  • Bonus recommendation from experimenting with VO2 measurement: try to focus on full, steady breathing throughout the interval. Starting to hyperventilate (faster, shallow breaths) is a sign that anaerobic metabolism is driving systemic response, and VO2 will measurably decrease. We’ll look at this in more detail soon.

Check out the comparison between these continuous VO2max intervals and 30/15s intermittent microbursts in the next post.

22 thoughts on “Comparing Metabolic Response to Hard-Start VO2max Intervals

  1. Hi Jem,

    first of all thank you so much for sharing such great insights. I have read almost all scientific papers on cycling that I have found over the last two years. However, coming across your blog has provided me with great new insights. I love the idea with hard start intervals and have personally tried the 30/15 on my own already before by starting harder in the beginning to get my heart rate up to > 90% of max as soon as possible. Those intervals worked great for me. But I have to say I never followed this idea of hard start intervals consistently and applied it to my continuous workouts, which I definitely will do now.

    I have a few questions to you:

    1) In previous posts with reference to Diagnose Berlin there was mentioned that time of heart rate > 90% is probably not a good indicator for time > 90% of VO2max. In an earlier post you say it is a reasonable approximation for many athletes. Given your experience with the Vo2master and other analyses now, would you say that heart rate is a good indicator for “ranking” workouts in terms of their effectiveness? I have compared several workouts and the 30/15 protocol gave me the highest time with my HR > 90%, so I was thinking that this might be a great workout for me. But obviously if HR is not a good indicator for VO2, this would be problematic.

    2) In terms of comparing continuous vs intermittent workouts with hard start, have you already come to a conclusion what works better and are there high individual variations between the athletes you have worked with? I think individualization is probably still an open point as even the most well documented HIT protocols in research also have few or several non-responders (although reasons can be broader like having not the best condition, being slightly ill, tired, personal issues etc). The ability to track time > 90% of Vo2max for different workouts might be a good tool to find out which workout protocols gives you the strongest signal for adaptation on an individual level. This might be 30/15 for me or continuous work for someone else.

    Thanks again and keep up with this great work.
    Cheers from Germany

    Liked by 1 person

    1. Thanks Fabs. Great questions.

      1) Check back tomorrow actually. I’ll have a post going up comparing HR & VO2 for the hard-start 30/15 microburst workout, and comparing back to this look at continuous power intervals (eg. 3x5min).

      From the minimal data I have, plus the Diagnose Berlin data, It seems that 90% HRmax correlates fairly well to 90% VO2max in continuous power intervals, but less well in intermittent microburst intervals.

      I say in the post that I suspect this is due to the faster on/off kinetics of VO2 vs HR, so that VO2 drops below 90% during the 15sec rests, while HR may remain elevated above 90%.

      Just to give the disclaimer, I haven’t looked at any actual performance outcomes of hard-start vs constant work rate intervals, or intermittent vs continuous.. which ultimately is the goal. I’ve only focused on time >90% VO2max as an estimation of adaptation. And I only have VO2 data on less than a handful of athletes undergoing these workout protocols. So extrapolate the conclusions at your own peril! 😅

      2) But from what I’ve seen so far there do seem to be responders and non-responders. In the sense that some athletes find it more difficult to reach 90% VO2max/HRmax rapidly (within 90sec) without blowing themselves up on the hard-start.

      My first thought like yours was that maybe these athletes were carrying too much fatigue or stress, or not enough carbohydrate fueling into the workout, or other transient factors. But it could be that these athletes have a combination of A) slower VO2 onset kinetics, so they take longer to stimulate their aerobic system up to near-maximal. And B) lower work capacity above CP/FTP, so they ‘deplete their battery’ before their aerobic system has time to ramp up.

      As I’ve been learning more about SmO2 and how to stimulate peripheral working muscle metabolism (mVO2), I think it should be possible to gradually ramp the workload up at the same rate as aerobic metabolism (ie. over ~90sec) to avoid the expenditure of anaerobic work capacity at the beginning of the hard-start interval. But then I guess it couldn’t be called a ‘hard-start’ interval any longer!



  2. This is a great blog. I was wondering why you weren’t aiming for 95% of VO2 max instead of 90%. I know Seiler finds 90% is manageable, but he prescribes over 30 min’s at 90% with his 4×8 min’s.


    1. Hi Todd, Thanks!

      I’m sure you’ve seen my many disclaimers and cautions about using HR to estimate VO2 🙂 The 90% HRmax target is inspired by Seiler, Rønnestad, Thevenet, and others. Seiler’s 2013 paper comparing the 4×4, 4×8, 4x16min protocol states that “Heart rate is based on the average heart rate over the last 25% of each work period” (bottom of Table 2, p.77

      So of the 32min (4x8min) this actually suggests less than 25% (8min) of that time was truly above 90% HRmax.

      With these hard-start decreasing-power intervals, you should reach 90% HRmax rapidly, typically within 90sec and maintain above that target as power decreases to account for fatigue through the remaining minutes. You’ll have seen in this and other articles that the real time accumulated >90% HRmax with these types of intervals regularly exceeds 10min >90% HRmax and >90% VO2max.

      One of the reasons why I don’t think using a 95% HRmax target is necessary for most people is that like most of training prescription, it’s better to hedge a bit low and complete the full prescribed duration of the workout, and with freshness to maintain high quality training tomorrow, rather than blow yourself up in a 10/10 workout that requires longer recovery time. This is the whole basis of the findings that greater duration at slightly lower near-maximal intensity, is ultimately more effective than less time at maximal intensity.

      That being said, there will certainly be people for whom this kind of interval training is appropriate at 95% HRmax target. And probably just as many who would be more appropriate at 85% HRmax. The tough part is figuring out which you are!



      1. Thx for the reply. I’ve used 90% of modeled pVO2 from WKO4 as a target. I’ve found that to pretty doable, but the last 8′ interval is tough. I agree that doing a little bit lower intensity means I can do a lot more work at a pretty high intensity without being destroyed. I’m prepping for ‘cross, so not sure if the 90% intervals on their own will be enough intensity. Great blog.

        Liked by 1 person

      2. Todd, take a look at an article I posted recently looking at measured VO2 compared to modeled VO2, if you haven’t already. Very relevant.

        Using WKO modeled VO2 can be a good way to get a starting power target for interval prescription, but just be aware how it’s limited. The model infers VO2 from power alone, so it’s unable to account for drifts in metabolic efficiency, ie. how VO2 changes through a single interval or an entire workout as you fatigue.

        I’m not an expert in CX training, but think about what kind of efforts will best replicate the demands you’ll encounter in your races. Might be something more stochastic, eg. even harder, shorter bursts above PVO2max on top of a sustained hard tempo? Good luck!


      3. Great article. I’ve wondered about the modeled time at % of VO2max chart. It doesn’t seem to capture the load of the on/off intervals in the 30″ range and, like you said, the ongoing intervals in a set are modeled the same even though it HR and breathing rate ramp up faster as the intervals progress.


      4. Exactly Todd. It’s beginning to bug me when power is used to infer VO2 during constant work rate intervals, when the VO2-power relationship was derived from a ramp test. A ramp test is designed to elicit a linear physiological response to a linear increase in workload. The dose-response relationship is predictable and an [over-]simplified linear VO2-Power relationship can be derived. This is valuable of course, but to extrapolate that linear relationship to all workouts I think is inappropriate and inaccurate.

        I suspect the ACSM model used in WKO was based on just that. It works very well to predict VO2 during a ramp test and for submaximal (<CP) intervals where the VO2-power relationship is (relatively) more consistent. But it appears to fail for intervals above CP, ie. ‘VO2max intervals’ of any variety. I’ll have some other interesting comparisons coming soon on that topic.


  3. Oh I kinda found what I was looking for regarding SmO2 and VO2max. “… more time at minimum %SmO2 (although I don’t have exact targets for SmO2 yet).” Any progress on SmO2 targets?

    Also, regarding fast/slow responders, if one can get to 90%HRMax with 90s, this is considered normal/fast responder vs slow or non-responder?


    1. Lin, you’re on the right track for sure.

      Last question first on Onset Kinetics: Traditionally ‘VO2 onset kinetics’ is discussed in reference to fast and slow responders, but HR and SmO2 onset kinetics will be related. How quickly you reach 90% is related primarily to the workload intensity – which is why hard-starts are used to rapidly reach that target – but for a traditional constant work rate ‘VO2max interval’ VO2 can reach steady-state within anywhere from ~30 to 120sec. See Poole & Jones 2012 as a great place to start reading about onset kinetics.

      I have to catch up on more recent literature myself. Check the citations, there might be some interesting recent developments.

      From the anecdotal data I’ve seen myself, well-trained cyclists have onset kinetics typically between 60-90sec for VO2, HR, and SmO2, with that time decreasing at higher intensity workloads, and increasing with acute fatigue during the workout.

      So simple answer: If you’re taking ~90sec to reach 90% HRmax, and I assume the initial rise in HR (Phase I + II) is therefore less than 90sec, I would consider that a perfectly normal onset kinetics response.

      Regarding what a ‘slow’ response might look like: I’ve seen a distinct significant delay in onset kinetics imposed by what I suspect to be ‘all-life’ or sympathetic fatigue: ie. an athlete who normally has fast onset kinetics of 120sec to reach equilibrium even at heavy intensity (below Critical Power). And worse, at severe intensity (>CP) his VO2 is so slow to respond that SmO2 de-saturates essentially to zero, causing task failure before he can ramp up VO2 supply to meet demand. ie. before VO2 or HR ever reach 90%. Very interesting implications for recovery and fatigue management in training periodization.

      I’ll follow up on your other questions on SmO2 after a quick ride today! 🙂


    2. Lin, regarding SmO2 targets for VO2max-type intervals, I would want to re-read some of the literature and confirm my understanding of the applications before giving a firm recommendation. But let me just wing it a bit off the top of my head.

      First, there seems to be a variety of individual responses in SmO2 to what I’ll continue to call ‘VO2max intervals’. So we don’t have a broad target like ‘time accumulated >90% VO2max’. SmO2 is especially sensitive to sensor placement (between different muscles or even on the same muscle belly), fiber type composition, adipose tissue, and some other factors. This makes giving a target magnitude (SmO2 in %) difficult between individuals. But it may still be possible to have a second-order target, ie ‘reach 20% of your individual SmO2 range’.

      SmO2 trends, ie how SmO2 responds to workload also seems individualized. Some people (or at least certain muscles on certain people) de-saturate straight to a minimum, then plateau for the remainder of the interval. While others show a continuously decreasing slope toward a minimum through the interval.

      Now this is all academic if you have a sensor of your own and can use it consistently during training. You can begin to describe your own individual response and can begin to prescribe training based on your response. For example, I’ve been using a Moxy on vastus lateralis muscle belly, in a consistent position using my tan line as reference (it’s basically a tattoo at this point 😅). At this location I know generally what %SmO2 I should be at during recovery, aerobic, or high intensity training. However my problem has been, I seem to just de-saturate essentially to zero for any workload above ~350 W.. Just looking at SmO2, I might be near VO2max at ~400 W which I could hold for ~5min, or I could be at 600 W which I’ll blow up after ~1min. So how can I better use SmO2 to guide my interval prescription if there’s no apparent difference at these workloads? Dunno.. open question

      Some people show similar behaviour, some don’t. I’ve been playing with different sensor placements more recently, and I might be finding some potential solutions to this issue. For example, placing the sensor more proximal (higher) on VL or on RF (rectus femoris on the quad) might offer better sensitivity to changes in workload.

      I’m very curious how Humon has programmed their app to account for this kind of individualized response. I wonder if it has some kind of ongoing calibration or machine learning to improve the prescriptive ability and colour coding as you use it more? If it doesn’t we might be back with the same limitation of recognizing individualized response vs population norms, like FTP. Just like 95% of 20min power might work for some/many/most people, an SmO2 slope of (eg.) -5 or target magnitude of (eg.) 25% might work for some/many/most, but not all people… Good thing is, the models will only improve from here!


      1. Thanks for taking the time w/both of your detailed responses. I like the way you think! I haven’t even got my hands on the Humon yet (ordering this week), but based on my own reading have many of the same questions as you regarding muscle choice, sensor placement, individual response, etc.. Worst case, w/consistent use and experimentation, hopefully one can identify a consistent individual response and thus prescribe appropriate training. I doubt there is a one size fits all solution to be found. However, I would expect (perhaps wrongly) that general guidelines/ranges/trends exist for the the majority.

        Liked by 1 person

  4. Hi Jem,

    I think the big question for me was something that you alluded in one of your first responses. That is, are there any outcome trials that demonstrate benefit with these declining power output intervals. Both the Billat [2013] and the Lisboa studies demonstrated that time at VO2 max could be greatly increased with this type of procedure, but neither included training outcomes relevant to real world practice.

    That raises the question at the centre of all of this. Is it actually time at VO2 max, oxygen deficit, or something else that stimulates mitochondrial biogenesis? The widely held assumption in the literature is that it is time near VO2max/O2 deficit the drives this. I think this is pretty likely, but there are other possibilities. For example some studies [again I think by Billat] that demonstrate a strong response to the actual intensity of muscle fibre contraction, rather than the oxygen deficit created. If this is the driver of mitochondrial biosynthesis, or at least as a partial driver then the declining power intervals might be less effective clinically then they seem to be in terms of their of VO2target time.

    I think this is an awesome experiment and I’m on the cusp of buying A VO2master myself! Very interested to hear your thoughts.


    Liked by 1 person

    1. Great insights William. Very true. I don’t think I’ve encountered any longitudinal performance outcomes from decreasing-power training.

      That same question also fascinate me: is time near VO2max the direct driver of adaptation, or is it just an easily measured surrogate for some other mechanistic stimulus? It’s important just to be aware that this ‘VO2max training paradigm’ is only one of possibly many simultaneously true paradigms.

      I’m currently down a rabbit hole reading about using cadence to manipulate training adaptations, inspired by Sebastian Weber and INSCYD’s training methodology for lowering VLamax. I recently encountered a super interesting idea: that ‘intensity’ can be distinguished from ‘workload’. Where intensity refers to %VO2max, and workload refers to power (W). See Tomabechi et al, 2018.

      The lightbulb moment was in realizing that cadence can be used to manipulate intensity and workload independently: eg. lower cadence at the same workload (same power) will elicit a lower VO2. Therefore at the same submaximal intensity (%VO2max) an athlete could train at a higher workload with low cadence, or vice versa, to optimize an appropriate adaptive stimulus?

      Again I think Weber’s ‘VO2max + VLamax paradigm’ is instructive, that some athletes will need to train at high %VO2max to optimize that signaling pathway, while others will need to train at higher workloads to optimize the VLamax pathway. This sounds like it’s getting at what you’re talking about. If you find that Billat paper, please link it!

      I highly recommend the VO2 Master Pro, obviously. It’s been incredibly useful for exploring ideas in my own training philosophy and for the athletes I work with. Peter and the team there are super helpful and there’s a growing community of very smart people experimenting and sharing ideas.


  5. Hi Jem,

    I like to call these hard start intervals RPE or lactate stimulating intervals. Reason being is that they are not the intervals with the most time in high power zones (‘PWR’) nor the intervals with the most time in high heart rate zones (‘HR’).

    Reason for this – most likely – lies with the fact that a supra-threshold/supra pVO2 (max avg power for 5 min) start taxes the anaerobic system significantly and builds a significant amount of lactate acid in the muscles. My routine is 2 minutes at 110-120% to pVO2@5 min followed by 100-110% to FTP. I generally manage two such sets with the second set often exceptionally hard Relative Perceived Exertion (RPE) meaning that I would not be able to do more than 5-10 more seconds at 100-110% to FTP at the end of the 8 minutes.

    Compared to HR or PWR stimulating intervals I am quite sure that these Hard Start RPE intervals are stimulating the body in different ways. For example PWR intervals are generally such that allows you to accumulate more time in higher power zones indicating less active to passive time, i.e. shorter intervals with longer rest time, e.g. 30:30 or even 15:30 etc.

    HR driving intervals are interesting. My experience is that the intervals where I can accumulate the most time in high (>90% or even >95% of max HR) are when the heart rate is slowly but steadily ramped up to the desired level e.g. >95% through increasing work load like in a ramp test. From there on work load is decreased – just like the hard start intervals – but maintained at a sufficiently high level so that the HR does not fall below the desired target (>95%). The main difference is likely that compared to the HS less lactic acid is accumulated (i.e. the aerobic component is larger throughout the interval) and hence the body can tolerate a continued work load at a very high HR. I am not convinced that these intervals really have a specific function – certain pro riders/teams seem not to care about HR but rather about RPE – but it makes it possible for me to accumulate up to +10 minutes above 95% of max HR which otherwise is impossble ( I am generally at around 90-94% of max HR on average for the RPE intervals at a Perceived Effort very close to absolute max (19,5).

    PWR intervals serve a distinct purpose by making it possible – through the right mix of active to passive time – accumulate significant amount of time in high / targeted power zones e.g. around pVO2, at specific anaerobic ranges etc.

    RPE intervals are in my experience exceptionally hard and race like and stimulates both the anaerobic energy system as well as the aerobic. Hopefully it also adapts ypou for convertic lactic acid into energy and builds mental toughness and pain tolerance.

    As a side note, I have measured lactate against perceived effort and the correlation is exceptionally strong why I like to call lactate driving intervals RPE.

    Would be interesting to hear your reflection on this.

    Best regards / Björn


    1. Hi Bjorn, thanks! RPE/lactate stimulating vs power intervals is a great way to think about this paradigm.

      I’ve been trying to write a post for a couple months now tying some similar ideas together. The idea of ‘workload’ – external power output – compared to ‘intensity’ – internal metabolic energy cost. And how one can manipulate either semi-independently using protocol like hard-start vs evenly-paced intervals, higher or lower cadence, and exogenous stimuli like heat, hypoxia, and nutritional periodization.

      Tied to that is the idea of ‘efficiency’ – broadly what intensity is required for a given workload – and ‘capacity’ – as in catalytic capacity; what magnitude or rate of throughput you are capable of, in terms of either intensity (energy) or workload (power). This comes from the idea that improvements to VO2max can come at the cost of decreased (gross) efficiency, and the mechanisms behind why (dive down the rabbit hole starting here: Anaerobic contribution to power output and lactate production/clearance/accumulation also plays a role. VLamax/VO2max relationship I think is one way of describing this efficiency/capacity trade-off, or maybe more accurately the aerobic/anaerobic components of that relationship.

      Under this paradigm, hard-start or RPE/lactate-stimulating intervals would force performing in a low-efficiency state, or metabolic environment. Where internal intensity remains elevated, while external workload declines compared to typical evenly-paced efforts. Hence why VO2 remains elevated near maximal at lower power outputs. For example I just performed a fun and painful workout yesterday ( comparing 8min 30/15s and continuous power intervals. Blinded to everything but time and paced as ‘iso-effort’, ie. trying to maximize power over the full interval. Some interesting results, given my pathological physiology.

      The question then is, how can we use these type of intervals to modulate our training stimulus toward either enhancing efficiency or expanding capacity? Is it possible to preferentially stimulate one over the other? Will training in a low-efficiency metabolic environment stimulate capacity at the cost of entrenching that lower efficiency? Or vice-versa, will a low-efficiency metabolic environment elicit improved efficiency at that capacity? Maybe both can be true under certain conditions or for different individuals? How do we know what those conditions or who those individuals are??

      To your point on HR, can the HR-power relationship (ie. internal-external) tell us anything about the underlying stimulus? eg. when HR gradually rises toward max, compared to a rapid rise and plateau? HR is influenced by so many factors, the information may be in there, but the same ‘time near HRmax’ probably won’t elicit the same adaptations, if that time is achieved via different protocol. Same thing goes for VO2.

      Lots of interesting questions I don’t know the answers for! I could rationalize in either direction. I’m still reviewing the literature and synthesizing all the info out there, which is why the article has been taking so long! 🙂

      Great discussion. Keep the ideas coming!


      1. Hi again Jem,

        Yes there are some complex relationships that you describe and it’s absolutely hard to say what will work in best in practice. Some focus on VLamax and there seems to obvious reasons for lowering it and gain aerobic efficiency but at the expense of anaerobic capacity.

        My thinking is that the “optimal” training regime will depend on your genetics (body and muscle composition) and your previous history of training – sprinters, puncheurs, GC riders are very different – and although some general advice will apply for all, certain specifics may be more or less efficient for some.

        For me, Vlamax lowering training just does not seem to work. I get tired, loose a big portion of my anaerobic capacity and it shaves off all of my sprinting (top end power) if I focus on that type of training. On the other hand when I do a lot of high intensity intervals – like those we are discussing, RPE, HR and PWR focused intervals – and pair that with some high volume low intensity distance training I get good results and the feeling is great. Then I am a typical “middle distance”/puncheur rider, without an exceptional sprint and without a very high FTP but with a generally good anaerobic capacity.

        Sorry for mixing personal observations in here but I tend to think that the hard start/RPE intervals are an important part of HIIT training as they are clearly quite different from other HIIT regimes (low-efficiency metabolic environment) which is exactly what you might experience in real race situation when break aways are happening etc. I kind of hate-love them as RPE intervals are terribly hard but afterwards are very rewarding.

        All the best / Björn

        ps. the Place where you did the 30:15 intervals is that where the UCI Masters Road World Championships 2020 are being held?


      2. “ps. the Place where you did the 30:15 intervals is that where the UCI Masters Road World Championships 2020 are being held?”

        If you mean the Gran Fondo world champs, yeah that’s being held from Vancouver to Whistler. Close to where I was doing those intervals. Hopefully it goes ahead this year…


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