VO2max is not always the limiting factor to performance. For beginner, technique improvement can deliver increase in endurance due only to the increased efficiency of movement form. Similarly, as you don't operate close to VO2max for long, other factors are important such as lactate tolerance, or even just psychological tolerance to distress.
This last point is particularly relevant for VO2max estimation, as a beginner is liable to feel that an effort is, subjectively, insurmountable, when objectively they still had metabolic reserve. Which will lead to overestimation of progress through initial underestimation of HRmax. I personally speculate this is what is behind the HIIT results, given its "taxing" nature.
So I would advocate for using as trainable number-go-up metric the normal performance figure of your sport of choice (minute per mile, rowing split time...), possibly supplemented with a percentile table for your age.
VO2max is not always the limiting factor to performance. For beginner, technique improvement can deliver increase in endurance due only to the increased efficiency of movement form. Similarly, as you don't operate close to VO2max for long, other factors are important such as lactate tolerance, or even just psychological tolerance to distress.
This last point is particularly relevant for VO2max estimation, as a beginner is liable to feel that an effort is, subjectively, insurmountable, when objectively they still had metabolic reserve. Which will lead to overestimation of progress through initial underestimation of HRmax. I personally speculate this is what is behind the HIIT results, given its "taxing" nature.
So I would advocate for using as trainable number-go-up metric the normal performance figure of your sport of choice (minute per mile, rowing split time...), possibly supplemented with a percentile table for your age.