Hey hey, we are currently recording a series of VO2max episode, and we would LOVE to answer your questions! 
Simply: reply to this thread!
Best, MJ
Should you be going āall outā each interval, or leave a little in the tank? Related, what are the differences in adaptation and stress between the two highest zones (6 and 7)?
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Thank @cgamio - added to our list 
My Tuesday vo2 max sessions are usually 30/30s. With a focus on a hilly xcm event are these the best versus, for example, 5 x 5, 40/20s and so on. And how would we know whether to āmix it upā using the workout wizard, or āstay the courseā by doing the 30/30s.
Love your work.
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Great Question @Phil . Added to the list
MJ
I have heard enough bits and pieces of the answer to this in the podcast and here on the forum to have a good idea of what the answer will be but I would love to hear it all put together (Iām still catching up on the podcasts, so perhaps itās in an episode that I have yet to get to):
For the running 30/30s, when picking out terrain, what is the most important aspect to consider? Is it that the interval be performed on a hill, as is suggested in the workout? Is it that the rest period be limited to 30 seconds? Is it something else such as consistency (all intervals on similar terrain instead of a mix)? What are the trade offs of doing 30 second intervals with 45 seconds rest to ensure the intervals are up hill vs. keeping the rest at 30 seconds and perhaps performing up to half of the intervals on flat terrain?
ā
Context: my area is near the coast and our terrain is small rolling hills. The biggest hill within jogging distance from my house is one which supports about 4 intervals (run 30s up, turn around, walk 30s down, turn around, repeat) before Iām at the top (since I travel further during the uphill interval than during the downhill interval). At the top I have maybe enough flat terrain for a 30s interval (100-150m) before Iām going down the backside of the hill and can do the remaining intervals on this flat terrain at the top. The alternative is to take 45-50 seconds for the rest interval to ensure I get far enough down the hill that I donāt reach the top before the end of the set. Or to use the ample flat terrain around me to ensure consistency in all intervals.
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Iāve recently returned to the 30 / 30 short HIIT format having been doing 5ā Long HIIT this past year on the bike. I am finding that the 30/30s donāt raise my heart rate as much as the 5ā intervals. Primarily because it recovers well during the 30 seconds off I think. Should this concern me, or donāt worry as there are other physiological benefits being induced?
Hereās a sample 30/30 HR trace from an outdoor execution. The new improved AI coach seems happy that I nailed it.
Same - I have a great hill at home thatās perfect for 5-7 intervals but any more Iād be going over the top; also when visiting family out of town itās super flat, with a tiny hill (aka a park that has barely-there-contouring). How best to do the all-outs on flat or descending w/o injury? Should we slow down?
Another unrelated question: when NOT to do 30:30s? E.g. when your HRV or sleep or recovery arenāt good, should we still do them? Pretty sure I know the answer from our live HIIT sessions, but would be nice to include some insights on when to call it and when to push it.