The featured athlete was training at very particular intensities. IME if one wants to do effective "low intensity" (ha-ha) training, it should be just below VT1. That's usually around 75% FTP or 80% HRmax I think one gets the most aerobic power increase right there. And remember, this is all HR data. As one becomes more aerobically fit, one's ;power at VT1 goes up and HR drift goes down, so more sustainable power. I think the term "low-intensity" is misleading. Yes, intensity on some numeric scale is lower, but the perceived intensity isn't all that low. On a long ride, say 9+ saddle hours, going as hard as I can, my average in-saddle HR will be below my VT1. I think that's quite normal. That would obviously not be the case in a crit, but the Couzens example is from an Ironman. That changes everything.