A fair question.
I only got myself a power meter (Assioma UNO, later upgraded to a DUO) about 3 years ago. Prior to that I was training with HR only, so I had put a fair bit of time into trying to understand the nuances of HR. Now that I have a PM, I tend to use power as the main metric. I am more of a sustained power kind of rider, so the lagging indicator bothers me less than it might for a short power rider.
The 3 main things I still look at HR for are:
- aerobic decoupling, sometimes as an indicator of aerobic fitness, and sometimes as a fatigue indicator on long rides.
- seeing elevated HR on shorter rides suggests that I am not fully recovered, slept poorly, fighting an illness, and an indicator that perhaps I should cut that day’s training short and head home.
- this afternoon’s ride was one of the first warm (32C/90F) of the (southern hemisphere) season, and I knew I wasn’t acclimated to the heat. It was only a zone 2 ride, and while I rode to the middle of zone 2 power, I kept on eye on HR to ensure it didn’t go above the top of z2 HR. In a few weeks when I am better acclimated, I’ll ride to power alone, but for now, I prefer to keep an eye on HR as well
- z1 recovery rides I tend to ride to HR rather than power. Inevitably I will have to go above z1 power to deal with short climbs, steep ramps, etc. If my HR creeps into zone 2, it’s a sign I have been working harder than ideal for a recovery ride for too long.
In short, while I definitely ride to power, I tend to use HR as a guide as to when to put a cap on things, when power alone doesn’t tell me how my body is doing.