More Comparisons for Hard-Start VO2max

I wanted to share a quick visual comparison of another athlete trying the Hard-Start VO2max workout. This might be helpful for comparing your own data in Strava and dialing in your individualized workout targets.

The athlete wasn’t sure of his precise power targets, so he plugged in something reasonable and gave it a go. After 3x reps he was satisfied that it was hard, but he was still able to produce an extra interval at the end for 4x4min. He reported the session RPE was 8/10. Not bad… but I thought he could he could further optimize this workout.

4x4min Hard-start VO2max workout #1

I noted that his HR was rising through each interval and peaking in the high 180’s, suggesting that VO2 slow component was causing HR to drift higher through each interval.

The intent of the Hard-start workout is to reach near-maximal VO2 as quickly as possible, and decrease power to compensate for the VO2 slow component (a decline in efficiency over time at severe intensity above CP/FTP). Meaning HR should reach near maximal and plateau as power declines, rather than drift up toward maximal at the end of the interval.

Without knowing anything else about this athlete’s HRmax or VO2max, I recommended he try to raise HR all the way into the mid-180’s during the 90 second hard-start. Then dial back power to maintain HR in the mid- to high-180’s

A week later he attempted the same workout, with what I think were much better results:

3x4min Hard-start VO2max workout #2

This time he extended the hard-start intervals until he reached 185 bpm (showing very impressive consistency) then dropped the power to maintain 185 bpm for the remainder of each 4min interval.

Session RPE was 9/10 for only 3x intervals this time, and average power was actually very similar for each rep. I would guess he accumulated relatively more time >90% HRmax & >90% VO2max in just 3x intervals, than he did in 4x the week previously. But it would be nice to confirm that with the VO2 Master Pro.

Interval Prescription Guidelines

If like this athlete you aren’t sure what your interval targets should be, basically just try something that sounds hard and see what happens. Then dial in your targets from there. The workout is nicely self-correcting in that way.

  • 90sec Hard-start: Try aiming for your recent 2min or 3min peak power (peak power in the last few months, not your all-time max from 2 years ago…)
  • It’s difficult to prescribe this workout based on (eg.) 120% of FTP, because of individual differences in eliciting and maintaining VO2max. Which is why we aim for a heartrate target and monitor RPE
  • Hold that power until just before you feel like you’re reaching exhaustion, or just before you begin to hyperventilate
  • Drop the resistance or shift down a gear and continue working
  • Continue to drop resistance or shift down until you find the balance of maintaining the workload with HR remaining plateaued near maximum.
  • Focus on full, steady breathing without hyperventilating
  • If you reach a point where you can no longer maintain your HR target, or you begin to hyperventilate. End the interval
  • I think 3-4x 4min is a good prescription to start with. If you can do that successfully for a couple weeks, go for 5min intervals. Extend the interval length rather than increase number of intervals

5 thoughts on “More Comparisons for Hard-Start VO2max

  1. I have said it before and I will say it again; really cool blog posts.

    I like that you try to break down the mystery of VO2Max intervals. My personal conclusion is that training is more individual thing than generic (or even specific) training programs can describe. I think (as many similar riders) I got a bit “obsessed” with training by watt numbers and zones.

    I don’t have the same fancy measurement equipment available as you do. But heart rate and RPE has again become important factors in deciding interval intensity, rather than just being a slave to raw watt numbers.


    1. Yeah Lars, (and thanks!) the more I look at SmO2 especially, the more I see how well it correlates to RPE most closely, and HR & Power only indirectly.

      I’ve been experimenting recently with SmO2-guided anaerobic (20-30sec) intervals, and pretty reliably I can say when I start to feel the burn, I’ve depleted SmO2 to a minimum. When I then back off and wait for SmO2 to resaturate it tells me I’m ready to go shortly after the burn goes away. Makes for pretty simple anaerobic interval prescription!

      See Strava for one of the experimental workouts:


  2. What do you think, is the extra time above 90% hrmax going to lead to more adaptations? I wonder if the faster VO2-kinetics aren’t due to a shorter cardio-pulmonary phase (typical delay between raise in muscle fiber VO2 and VO2 measured by mask) caused by higher central stimulation instead of a faster muscle adaptive phase? And are, because of this, constant intervals not just equally effective in enhancing endurance capacity?


    1. Hey, thanks NF. Good line of thinking on this topic.

      I don’t actually know how responsive the cardiodynamic phase (Phase I) is specifically to changing intensity. I’ll have to look into that. But I definitely know the rate of the primary phase II increases with increasing intensity, at least up to MAP and the transition from severe to extreme intensity domain. So generally speaking a higher intensity start at interval onset will elicit faster pVO2 kinetics.

      And from working with NIRS and muscle oxygenation, I can confidently say that higher intensity also elicits faster mVO2 kinetics, again up to a point. So while there are certainly other factors involved, I see a direct link between increasing workload during a hard-start, increased rate of muscle oxygenation kinetics, and increased rate of pulmonary VO2 kinetics.

      We’re basically trying to increase the rate and amplitude of the primary component of VO2 onset kinetics, in order to reach higher %VO2max, faster. Then decreasing power to mitigate the VO2 slow component bringing us to 100% VO2max and task failure. Whether that comes from Phase I or II I think is less relevant?

      As for do I think this pacing manipulation will lead to greater adaptations? I thought so at the time of writing, from a speculative/theoretical standpoint. I still see a rationale for enhanced adaptive effect. But if I had to bet money, I’d probably bet less now than I would have when I wrote this. Based on theories about where the ‘excess’ VO2 is going, if not directly to locomotion/power output.

      Will that ‘non-locomotive’ excess VO2, and therefore performing these intervals at lower economy (W/L/min) have the same adaptive effect as an evenly-paced interval? I don’t know. Maybe it depends if you’re trying to optimize for peripheral effects (eg. mitochondrial biogen., angiogenesis, etc.) or central effects (eg. increased stroke volume & cardiac output) ?

      At the very least, I think hard-start intervals can be more race-specific than even-pacing.
      Definitely some interesting questions! Thanks!


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