I’m sure someone has already brought up the good advice that 120% FTP is not necessarily the universal intensity to sustain VO2max intervals, and that FTP itself is an estimate of our max metabolic steady state, which is some theoretical moving-target construct that will change day to day and minute to minute.
The first priority in my opinion for ‘VO2max’ or high intensity interval training is that the work should be performed within the severe intensity domain, meaning above FTP / CP / MLSS / whatever protocol-specific estimate of max metabolic steady state that we want to use.
Saying intensity should be above sustainable steady state isn’t necessarily the same thing as saying power must be above FTP / CP / etc. Efficiency / economy can change during high-intensity exertion with increasing duration, such that O2 cost of power output changes. Coming up at our threshold from below can elicit a very different response (and feels very different, obviously!) than coming back down to our threshold from above. When we’re coming back down from a higher workload, having disrupted metabolic homeostasis, our max metabolic steady state (construct) could now be lower than our FTP number (protocol-specific estimate), See the recent narrative review from Maunder & Seiler et al which touches on all this.
That being said, if power doesn’t follow a strict proportionality with VO2, neither does HR. So maintaining eg. 90% HR doesn’t necessarily mean we’re at a consistently high %VO2max either. You could be starting too hard and drifting below true max metabolic steady state due to fatigue, and HR could remain elevated due to other factors.
Again, the priority (IMO) is accumulating continuous work duration in the right intensity domain. We each have to triangulate what that means for us based on the data we have available. Including sensations / RPE! Remember, our brains are pretty good at integrating all these sensory inputs and predicting how hard we can go! HR, power, VO2 are all sensitive to multiple factors, so as people have suggested, we might be best served by just kinda aiming for a wholistic sense of ‘hard’ for whatever interval duration we choose.