We all respond differently to training. If you need 130% FTP to reach >90%FTP that is fine. I bet there are someone that feels 115% FTP is enough. Neither FTP nor HR are exact values; they vary from person to person and day to day.
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I started doing these this week. I think “hard start” is the wrong description. I’m calling these the Hampster wheel intervals. 
The first few times I went out too hard. Now I just focus on cadence and don’t even look at power too much. If I get it up to 115-120rpms my HR gets up above 90% really fast. If my HR falls a bit I turn up the rpms even if it means going to an easier gear. My 5 minute intervals have been coming in right around 120% of FTP.
So far I’ve only made a little over two 5 minute intervals. The first time was two failed attempts and two at 5 minutes for a total of 12 minutes in zone. On my second workout I made it to two 5 minute and one 3 minute intervals for a total of 13 minutes. And this was 48 hours after the previous workout so the legs were still feeling it.
Twelve minutes actually feels like a good stimulus the first time trying these. I’m sure I’ll build up to 3 intervals and then more.
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@AJS914 Yes, the hard-start or spin-up intervals are very challenging, but really help elevate HR and really initiate quick diastolic filling, which helps increase stroke volume. Good things for training VO2max. I find that if I do a normal start to these intervals at say 100-105 RPM it takes at least one minute to reach VO2max (or what I think is VO2max). Whereas spinning it up to 130-135 RPM gets me there in half the time. Which equals more time in zone.
PSA: If you’re feeling confident about your VO2 max planned for today, don’t read this thread 
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This is the answer, specifically because VO2 is not a power target. So everyone is going to be different when it comes to trying to sort out what % of FTP gets them into VO2 and over what duration.
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Here is how to find out for certain sure…take your resting heart rate. Subtract that from your peak cycling heart rate. That’s your heart rate reserve (as you probably know!).
No go do an interval that’s hard enough to raise your heart rate to 95% of your heart rate reserve. If you get to that point you are almost certainly in that recommended VO2max training zone. That’s what it feels like!
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I don’t believe this is true. Example:
MHR: 195
RHR: 65
HRR: 135 ( = 195-65)
95% of HRR = 128 BPM
That’s certainly not VO2max
Sorry…I should have actually done the math for the (probably majority) audience that is unfamiliar with the concept of heart rate reserve (HRR). I’m trying to walk the line between patronizing and informing. 
Let’s say peak cycling heart rate is 185. Let’s say resting heart rate is 48. HRR is 185-48 or 137. 95% of HRR is 137*0.95 = ~130. So if you want to be pretty sure your in that 90%+ VO2max training zone target a heart rate of 48+130 = 178.
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one more step… you have to add RHR back in:
- 128 is 95% HRR
- 65 is RHR
- 193bpm target heart rate at 95% HRR
- 187bpm target heart rate at 90% HRR
Then add RHR back on to give target of 188…and HRR is 130 in this example.
Totally agree that your method would certainly put you in VO2max.
Something that hasn’t been mentioned in this thread is the importance of being well-rested before a VO2max workout. It’s imperative.
If you’re carrying an amount of fatigue that would prevent you from a max repeatable effort you’ll probably have trouble actually achieving the stimulus you’re after.
HR is only useful after the workout IMHO. VO2max work should hurt and be max repeatable effort, whatever that is, and not looking at a computer and basing on a HR target. Over time you learn about your personal power and HR responses to vo2max work.