My confusion is what happens during times of transient power and HR, and how that gets allocated to theoretical stress buckets. Sure, power and HR zones correlate well when looking at efforts that reach a steady-state heart rate. But thats not true when power changes and HR is rising or falling as a (delayed) response.
E.g. from sitting on the sofa, to jumping on my bike and pedalling, i can hit zone 3 power (say 110% TP?) in a few seconds. Now while holding that power, it takes my cardio system a few minutes, starting from resting HR, to finally reach a steady state value that is what i would classify as my true zone 3 HR.
Now, from a stress/stimulus point of view, my legs and muscles feel every second spent at that zone 3 power (and maybe more for no warmup!). However, my cardio system doesnt feel the same level of stress (and i suspect training stimulus?) until the HR is up in the (steady-state) Zone 3 HR zone.
So under transient power - and with the intent of training your aerobic system - it surely has to be HR that reflects stress and training stimulus the best. And therefore HR is your best guide if you are trying to minimise zone 2, and maximise, zone 1 + 3 time (for transient workloads)?