Why Perform Hard-Start VO2max Intervals?

Hard-start, or fast-start intervals use an initial hard effort at a power output above what would be sustainable for the intended interval duration, to enhance oxygen uptake (VO2) and cardiac output (Q) in order to better match the total energy demand of the entire interval with oxidative metabolism.

As the athlete rapidly approaches VO2max, power is decreased, or tapered to allow the athlete to maintain an elevated VO2 for the entire remaining work duration, without reaching task failure. Getting to VO2max quicker means greater time accumulated near VO2max for the same total interval duration, and for the same or less total mechanical work (kJ) performed.

In this way, studies investigating hard-start pacing strategies have shown that metabolic intensity (time near VO2max) can be increased independently from mechanical workload (power output) within an interval workout. (1-4)

Why Even Talk About Hard-Start Intervals?

Before we go on, let me be clear: I don’t think hard-start intervals are any kind of magical ‘last and only interval you’ll ever need to do!‘. Just like any other single workout, they are only one piece in the more important macro structure of your overall training plan.

I’m not sure that hard-start intervals are the optimal, or potentially even as good as traditional evenly-paced intervals.

Hard-starts might not be as efficacious on paper to elicit improvements, but I do think they can be effective when used appropriately. And I have seen good individual responses to including them in certain training blocks.

However honestly I just find the question of interval pacing a fascinating paradigm in which to explore and learn more about the metabolic response to exercise. That more than anything is why I keep finding reasons to write about interval workouts!

Let’s talk about some of the reasons and rationale for why we might want to perform hard-start intervals, when they might be appropriate for our training, and when they might not be. And I will give some advice on how to individualize hard-start interval programming for yourself.

Enhanced Oxidative Uptake

The primary benefits, in my opinion, of hard-start intervals is their ability to elicit greater duration near VO2max (↑ t@VO2max) and elevated total oxygen uptake (↑ total VO2), which are repeatedly suggested (if not conclusively demonstrated) to be related to adaptations toward improving VO2max.

This is primarily achieved by the hard-start workload stimulating local muscle O2 uptake (mVO2) and both systemic Cardiac output (Q) and local O2 delivery (mDO2), and thus speeding the overall VO2 onset response.

Hard-start 3min intervals. An example of starting the first interval waaaay too hard! And how the subsequent reps can be adjusted to preserve exercise tolerance and balance the cost of fatigue from anaerobic energy expenditure (pink area) and the stimulus to VO2 (blue area).

See below for advice on individual prescription of Hard-Start interval programming

Priming VO2 Response

We can think of the hard-start like a built-in ‘priming effect’ for each interval, where the metabolic response is enhanced by the harder effort, as mentioned above. For this reason hard-start intervals may be programmed different from evenly-paced intervals, in ways that might be more tolerance or achievable. (5)

For example, we can take longer and/or easier rests between work intervals, because we don’t need to keep VO2 elevated as high during the recoveries. Meaning we can better restore our metabolic milieu (the intramuscular disruption from anaerobic energy expenditure) and perform our subsequent work intervals with higher quality. Besides that, there are obvious psychological/motivational benefits of having easier rest intervals.

Another added bonus could be potential race-specificity of performing high intensity intervals scattered during a longer aerobic endurance ride. Because each interval is less dependant on having a priming effect from the interval before it.

Race Specificity

Race-specificity is another, less physiological, more performance-based advantage of the hard-start intervals. I’d suggest that most competitive endurance events are composed of at least a single, and more often repeated hard-start efforts.

Mass-start cycling races are clearly intermittent and ‘surgey’ in nature. But even TTs involve a primary hard-start off the ramp, and further surges around and over various obstacles of terrain (getting back up to speed after a corner is a perfect example). Middle-distance running, rowing, and probably other events I’m not aware of follow a similar hard-start pacing strategy.

I don’t think I need to belabour this point. I think hard-start intervals are specific and relevant to racing situations. (6)

Decreasing Workload

Here is where we get into potential downsides, or at least compromises to hard-start over evenly-paced intervals. Compared to a traditional evenly-paced interval, hard-start intervals will typically be designed to start at a higher workload, and to taper down to a lower workload during the latter steady-work portion of the interval. Depending on the length of the interval, the average power and total work (kJ) might be higher, equal, or lower.

While systemic VO2 is maintained near VO2max, a tapering power output may result in muscle fiber de-recruitment through the interval. This means fewer of the larger type II (faster) motor units are being activated, and so fiber-specific adaptations may be compromised. (7)

(Vanhatalo et al, 2011)
Tracings of power (A: top) and VO2 (B: bottom) showing faster VO2 response in a 3-min all-out test, compared to a constant work rate (CWR) 3-min interval, both programmed to perform the same total work (kJ).

INSCYD has suggested that larger muscle fibers & motor units need to be recruited in order to adapt their oxidative & glycolytic capacity (VO2max & VLamax, respectively). Iñigo San Millán has also pointed to muscle fibers needing to be activated and producing work in order to increase glycolytic capacity.

Larger and ‘faster’ muscle fibers will be progressively activated at higher power outputs, ie. during the hard-start but possibly not during the later tapered-work portion of each interval. So with fiber de-recruitment during hard-start intervals, it may be that fibers contributing less to the total power output will experience a lower adaptive stimulus. (8)

(Vanhatalo et al, 2011)
Tracings of electromyography (iEMG) in the same 3-min all-out and 3-min work matched intervals as above, showing decline in iEMG and suggesting de-recruitment of motor units with the all-out start.

Furthermore, the ‘excess VO2’ during the tapered-work portion (greater VO2 relative to lower power) could be attributable to increased ventilation and work of breathing. This ‘non-locomotor’ VO2, while enhancing apparent time near VO2max, may be suboptimal for eliciting the same adaptations as an equivalent duration from an evenly-paced interval. Where more of the energy derived from oxygen goes directly to the working muscles and producing locomotion. (9,10)

Prioritizing Central Cardiovascular Stimulus

This incomplete fiber recruitment and non-locomotor VO2 could actually be an advantage in other situations: It is fairly accepted that the primary adaptations to increasing VO2max arise from ‘central’ components of the cardiovascular system. Primarily stroke volume (SV: how much blood the heart pumps on each beat) and the O2 carrying capacity of the circulation (blood plasma and red cell volume).

Therefore, from a central perspective it could be less important where the Q and blood flow is ‘going’ and where the VO2 is coming from, and more important that a large volume of blood is circulating, and oxygen is being extracted somewhere before returning to the heart. This rationale has been used to explore varying cycling cadence, standing, simultaneous upper & lower body exercise, and other modalities that enhance full-body Q & VO2, in order to enhanced VO2max. (11-13)

It bears mentioning that enhancing VO2max is not necessarily the same as enhancing performance. Increased VO2max may not translate to performance outcomes. A colleague of mine, Michael Rosenblat, is currently completing his PhD on this subject and has published some fantastic meta-analyses on interval training and polarized training effects on cycling time-trial performance. He discusses the implications of this disparity in-depth in his PhD proposal, currently under review, that I’m eagerly looking forward to being able to write about!

Another important advantage could be that lower power output during the tapered-work portion of each interval might allow longer sustained work duration, in addition to greater time near VO2max. Fewer intervals could be performed at longer durations, and greater interval oxidative work could be performed relative to external mechanical work (kJ). (14)

Once again this depends heavily on how we program our intervals: whether to enhance time near VO2max with less work; to maintain total work over longer cumulative work duration; or to increase aerobic energy contribution toward total work.

Longer high intensity interval duration (≥ 4 minutes) has been shown to be related to enhanced cycling time-trial performance, and greater total high intensity work performed is probably another contributing factor. (15)

Auto-Regulated Effort

However, hard-start intervals may only be sustained for longer than the equivalent evenly-paced interval if they are tolerable. As mentioned above, the work duration and total work performed may be higher, equal, or lower, depending how the intervals are programmed, and on their perceived effort (RPE).

Different studies have found either increased or decreased exercise tolerance (lower or higher reported RPE) of hard-start compared to evenly-paced intervals, depending on study methodology. So this might be either a limitation or an advantage, depending on how we modify our own intervals to improve tolerance. (2,16)

I would suggest that this allows us to take greater control over our training by encouraging awareness of our sensations and perceived effort, and modulating our effort day by day, interval by interval. I still believe that RPE is the best real-time indication of training load and fatigue.

Our brains might not give us a precise number or clear yes/no signal when we have achieved our optimal training dose and could use some rest. But in general our brains are pretty good at integrating all the various stimuli that contribute to performance and fatigue.

Preserving Muscle Fatigue

The final possible advantage of tapered power output is that hard-starts might be used when our legs are fatigued from prior training: In a periodized plan, we could optimize at different times for peripheral or central adaptations, including multiple training modalities, and importantly including strength training.

After a strength training session, or after a race, DOMS and peripheral fatigue might last a few days and otherwise interrupt the training schedule. Hard-start intervals could be used to boost the central cardiovascular stimulus, while the tapered power output would preserve our legs more than an evenly-paced interval workout.

Hard-start intervals could be combined with sprint interval training (SIT) within a training block, to specifically apply more-central and more-peripheral training stimulus, respectively, as appropriate. (15)

Interval Programming for ‘Faster’ or ‘Slower’ Muscle Fiber Typology

A recent series of papers and follow-up articles have explored muscle fiber typology, or phenotype, in the context of recovery and fatigue imposed by training load. The perspective being proposed is that a very different functional response to a training dose occurs in athletes with ‘slower’ or ‘faster’ muscle fiber typology.

More ‘anaerobic/faster typology’ athletes can produce higher power outputs, but experience greater decrements in performance due to fatigue, and require longer recoveries than more ‘aerobic/slower typology’ athletes. (17-19)

(Lievens et al, 2020)
We can apply the lesson from Lievens et al’s work to demonstrate how hard-start intervals can be individually programmed to find an optimal balance of stimulus and fatigue for each individual athlete. And how the hard-start intervals might change throughout the workout, and at different phases of the training plan depending on fatigue and intended adaptation.

With this perspective we can discuss how to individualize our workout protocol, based on our own phenotype. No biopsies necessary: we probably know whether we are more ‘anaerobic/faster-typology’, like a sprinter or punchey athlete with a massive work capacity above Threshold. Or if we are more ‘aerobic/slower-typology’ as a diesel, TT’er, or ultra-endurance athlete who can’t attack to save our lives but can push steady power all day.

Individualizing Hard-Start Interval Programming

TrainerRoad user DaveWh has created some Hard-Start Interval workouts that provide an excellent template to follow, and further modify to fit individual requirements, as suggested below. Note: I DON’T recommend using Erg mode for these types of workout. See Empirical Cycling Podcast for a great discussion of advantages/disadvantages of using Erg mode.

  • Probably the most important advice is that the entire work interval MUST be performed ABOVE Threshold (FTP/CP/LT2/etc) in the Severe intensity domain. We can balance our individual workout parameters to be shorter and harder, or longer and easier, as suggested below. Just as long as the workload always stays above our individual Threshold.
  • A more ‘aerobic/slower-typology’ athlete can probably program the hard-start at a lower power target above their Threshold and/or for a shorter hard-start duration. Because they likely have faster VO2 onset kinetics and their Threshold is already at a higher percentage of their VO2max. Meaning they need less of a stimulus to get near VO2max.
  • A more ‘anaerobic/faster-typology’ athlete may need to perform their hard-start at a greater percentage above Threshold and for longer, for the inverse reasons as above. They will typically have a larger work capacity above Threshold, meaning they will need to perform more work to get an equivalent stimulus.
  • In general I would suggest that hard-starts do not need to be performed above max aerobic power (MAP), which is the maximum power at which VO2max will be achieved, in the shortest duration (~2min). VO2 kinetics may reach a plateau at MAP, meaning further anaerobic energy expenditure in the Extreme intensity domain may not be compensated for by the oxidative system, and may simply impose unnecessary excess fatigue.
  • Alternatively, the important criteria might be more about the magnitude and timing of work expended above Threshold in the hard-start, rather than the peak power output. We want to strike a balance between expending anaerobic resources to stimulate VO2, and not imposing too much of a fatigue cost for the subsequent interval.
  • Hard-starts can be less-hard/shorter with each subsequent interval repetition, because the previous efforts will continue to have a ‘priming’ influence on subsequent VO2 kinetics, and fatigue will mean less work capacity is available to perform above Threshold.
  • The tapered-work portion of the interval is also important to consider. More ‘aerobic/slower-typology’ athletes may be able to perform sustain a longer work duration. But they may not be able to increase the workload too high above their Threshold.
  • More ‘anaerobic/faster-typology’ athletes can likely sustain a higher percentage above Threshold, but may be limited by duration. As mentioned above, I would try to prioritize longer (≥ 4 minute) work durations, which may require these athletes to perform closer to their Threshold power.
  • If heart rate is decreasing during the tapered-work portion of the interval, we might want to make the hard-start easier/shorter, and the tapered workload harder if possible, and shorter if necessary.


This is my current advice based on my understanding of the mechanisms at work, and my experience with applying hard-start intervals to different athletes. Consider this loose advice on how you might be able to make the most of your high intensity interval training. And you should experiment further to find what works best for you!

I love digging into the details, looking for optimizations, and understanding the nuances (“there is never less nuance!”) of workout prescription. However the training prescription for the athletes I work with are usually far more basic.

Instructions for a hard-start workout might simply be: “Attack hard at the start, then back off and hold 8/10 effort for 5 minutes. Keep your power above Threshold, and prioritize completing the full workout duration. Challenge yourself and enjoy!”

Again, I’ll repeat that hard-starts have their place and I think are worth trying out. But the most important thing is to just do the work over the weeks and months, rather than get too caught up in the minutiae of a single workout or interval bout. And aim to understand why we are training the way we are.

The better we understand why we’re performing a certain workout, or training block, the better equipped we’ll be to adjust on the fly to improve our training experience.


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10 thoughts on “Why Perform Hard-Start VO2max Intervals?

  1. Very nice write-up. I like the concepts illustrated here.

    It’s a tricky balance, I think, to know what drives performance more: metabolic duration at Vo2 or greater mechanical loading…
    Also, rhetorically, how much should a coach/rider reinforce the already developed abilities in riders (e.g., easier & shorter hard-starts for aerobic athletes, and vice versa for more anaerobic athletes) versus specifically prescribing the complement to their abilities by working on the opposite type of hard-start?

    Lastly, it’s hard to forget the positive adaptations reported by Stepto and Hawley et al. circa 2001 when all athletes improved steady (40k TT) performance with 12x repeats only—no tapered intense work within the interval—at intensities of 175% MAP.
    Maybe keeping Supra-MAP hard-starts entirely separate from Vo2 work yields a better desired balance.
    My $.02.
    It’ll be fun to see MA Rosenblat’s results.


    1. Thanks Nate, great discussion points.

      That question of whether to train limiters or train strengths is a good one, and it’s probably an application of the ‘art’ of coaching more than the science. It takes more time and focus to affect change to an athlete’s limiters than to their strengths, so I find in my own coaching practice that we inevitably spend more time focusing on limiters, primarily during pre-season training. But the ‘it depends’ answer probably depends on where the athlete is in terms of current capabilities, where the athlete’s goals/event-demands require them to be, and how long they have in between.

      I’ll be curious to follow the ongoing work categorizing training response by muscle fiber typology from Lievens & Bellinger. I suspect they are working towards a more systematic approach to training individualization in this context.

      Are you referring to the Stepto et al, 1999 study? (https://pubmed.ncbi.nlm.nih.gov/10331896/) Yes they found enhanced TT performance with 12x 30-sec @ 175% Wpeak (peak power from an incremental ramp test), which is basically an all-out sprint interval training (SIT) workout. But they found the greatest performance improvements to both TT and Wpeak in the 8x 4-min @ 85% Wpeak HIIT group. Rosenblat looks at this study in particular in his previous meta-analysis on SIT vs HIIT (https://pubmed.ncbi.nlm.nih.gov/32034701/) and it will certainly come up again in the context of how SIT and HIIT interact from a peripheral vs central adaptations perspective.

      To your point on keeping SIT (ie. extreme domain exercise) and HIIT (severe domain) separate, that’s another outstanding question we have on training plan periodization: when and how much do we focus on peripheral adaptations, which appear to be stimulated with greater effectiveness by SIT, and when/how much do we focus on central adaptations, ie. from HIIT? Do we mix the stimuli simultaneously in the same training block, or do we focus on one at a time? If SIT is like sport-specific resistance training, could there be an inhibition effect from the combination of both training stimuli? Or as I speculate in this article, could you alternate application of separate and synergistic peripheral/central stimuli with clever fatigue management?


  2. Hey Jem, great article! I just “found” your page and there seems to be a lot of really interesting content.

    I am a sports science masterstudent and have just conducted an investigation comparing two types of interval sessions with regards to their acute physiological response in well-trained cyclists/triathletes (t>90%, Lactate, HR, Power, RPE, muscle oxygenation). The “new” approach in this study was that we compared the best effort/best average approach to a fixed intensity (based on % of MAP, as previously done in similar studies). The intervals were designed to be time-matched. So we had 3x13x30/15s (IM) with 6:15min set recovery and 6x5min (LC) with 2:30min recovery. In this approach one time 13×30/15 IM’s were treated as one interval, so that the 15sec are included in the hard time even though they are performed at a lot lower wattage. This gives an overall hard time (interval time) of 29:15 for the IM’s as well as the LC (last interval there only 4:15) with a rest of 12:30 for both sessions.

    And the real interesting thing here is that the average power (remember IM’s are treated as a set here) was nearly equal between the two sessions when using the best effort approach. I don’t want to dive more into the details (publication will hopefully be possible someday) but the physiological response was also fairly similar in the best effort approach between IM and LC. Based on my own experiences as a coach and athlete I actually suspect now no real differences between different forms of interval designs/sessions if the sessions are time matched (IM, longer/ shorter VO2max intervals, fast start etc.). As long as the pacing is very accurate the average power and thus physiological responses are similar.

    Obviously getting the best effort approach right is one of the challenges here. But with this trained participants it became clear that if they are given a fairly accurate instruction (“go as hard as you can, as steady as you can”) and accurate target power in the beginning their feeling (as you described in this article as well) works very good and they do maintain a very even pace throughout all intervals/sets which is crucial. So it might need one “test” session to get a feeling which power is a good target but then already the second time you perform the workout it’s as efficient as it gets because you are maxing out your physiological capacities – no matter how you design the session. This might be a “hot take” but I encourage people to try it out for themselves. You might be surprised how equal the average power is, even when the design is different (remember to time match though!).

    Sorry for the long post. But I would love to hear your perspective on it and especially with regards to these fast start intervals. I actually suspect them to be equal in physiological response (mainly t>90%) as well and that they won’t be different to continuous ones if both are performed using the best effort approach (with fixed intensities it was already shown that they work better but I think the best effort approach is the great “equalizer” here). From our study it seems that the fixed intensities (based on calculations like MAP/threshold etc.) are creating different physiological response between different protocols.

    Liked by 1 person

    1. Daniel, thanks for reaching out. I’m gonna email you. Lots of thoughts 🙂

      Your approach sounds very similar to a training study design I’ve been working on with a colleague. We have also come to think that the ‘dose’ of training stimulus may be more related to total time or even just sustained work bout time ‘in a disrupted metabolic milieu’, as an important correlate to performance improvements. Similar to how time near VO2max is correlated with VO2max improvement. eg. Comparing total work-matched and total ‘work time’-matched short intervals (<4-min) vs long intervals (>4-min) vs intermittent sets (13×30/15s, considered as one set or bout).

      As for the best-effort HIIT pacing, super interesting! And from a coaching perspective I completely agree there is added benefit from learning the pacing necessary to maximize self-regulated interval training like that. I often repeat that our brains are good at knowing where our threshold is on any given day; and if we learn how to pay attention to our sensations, we should be able to pace a best-effort training session to get the most out of our performance, without overdoing the metabolic strain. I love to hear that your findings seem to support this empirically! I look forward to reading the published study. Sounds like an important finding.

      One of the athletes I train with just talked about a recent interval workout he performed (which wasn’t deliberately hard-start, but it ended up that way 🙂 . He said he attacked hard because he was feeling good at the start of each interval, and ‘rode the wave’ of almost blowing-up back down for the remainder of the 6-min efforts (long intervals, I know!). Each bout looked consistent, so I consider that an excellent self-regulated workout.


    1. Hi Luca, sorry for missing this earlier.

      I think ~90-seconds makes sense for the hard-start – that is, power starts higher and tapers lower toward your target steady power over the course of ~90-sec. This is just a loose idea based on the time course of pulmonary and muscle VO2 onset kinetics during a steady interval, which stabilize somewhere around 60-120 sec, depending on the individual, the intensity, prior efforts, and so on.

      So through that first ~90-sec you would be tapering power at around the same rate as VO2 is rising. Like you’re trying to have work output and O2 delivery meet in the middle.

      Hope that makes sense.


  3. I think, it’s time to reevaluate the thesis “higher time at high vo2 percentage is better for vo2max gains from hiit”. I do not understand how Ronnestad came to this conclusion, because the corresponding experiments were not carried out. It is necessary to give a group of people the opportunity to make the same type of intervals with a gas analyzer, and build correlation models. I think, we should also find signals generated live during a HIIT session, which are indicative of response probability. Maybe, lactate, thermogenesis, smo2 drop, time at high vo2 percentage (or low time during a hard effort), spo2 drop


    1. Hi Denis, thanks for the comment.

      I completely agree we should be mindful of the assumption that time above X% VO2max produces improved adaptation outcomes. But it wasn’t Ronnestad’s group that came up with this idea. And indeed as you say, that wasn’t the research question they intended to explore.

      I actually went digging around into this claim a few years ago. As best as I could figure out at the time, the claim of ‘time >90% VO2max’ originated from a 1986 review article by Wenger & Bell in which they descriptively (not statistically) compared previous training studies reporting changes in VO2max among their participants, along factors of training intensity, frequency, duration, programme length, and starting fitness level. They found that there was an optimum in the pooled results around a training intensity of 90-100% VO2max.

      Wenger & Bell, 1986

      It was a reasonable interpretation at the time. However, I would also love to see a modern controlled experiment challenge the same research question, or a meta-analysis designed to synthesize the contemporary literature on this question.

      That being said, there are plenty of contemporary experiments, including some by Ronnestad’s group, that when considered together (interpolating between acute responses and longitudinal outcomes) continue to support an association between accumulated time near VO2max and improved VO2max and/or performance outcomes. It would be great to mechanistically elucidate the relationship.

      My speculation is that the volume (intensity x duration) matters more than pure intensity per se. A colleague’s recent meta-analysis suggested that improvements to time-trial performance from interval training within the severe domain was independent of intensity – that is, as long as the intensity was in SVR domain, the relative intensity did not matter. Longer duration intervals had a greater effect on performance.


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