[Scott Johnston]

You bring up some very good points.  As you point out; there is no simple answer.

For good measure you could toss in the intensity at which blood lactate has its first appreciable rise (or rises by 1mMol/L according to some sources).

There is even a newer method of defining this point using HRV.  I’m not familiar with this but as I recall: By noting when HRV decreases by, I think 25% this corresponds closely to the gas exchange test for VT1.

The physiology definition of VT1 (first ventilatory threshold which many would call the aerobic threshold) is the intensity at which the rate of ventilation (volume of air inhaled and exhaled in a minute) increases fasted than than the rate oxygen is extracted by your muscles (VO2).   Up to that intensity these two markers increase in lock step. Beyond that the ventilation increases faster than the O2 extraction.  This point is hard to find without a gas exchange yes in a lab. Ventilatory markers of conversation and nose breathing only roughly correspond to it and vary quite a bit from person to person.

Your observation that nose breathing and conversational breathing paces vary a good deal from day to day is consistent with what I have said for years. It depends on your recovery status and your training modality.

I can’t say much more definitively except that we have seen pretty good agreement between the gas exchange test and the HR drift test. Beter than the perceived ventilation markers of nose breathing and conversational pace.

Scott