Power vs. HR on TrainerRoad

not sure, might have something do to with your aerobic base.  i'm actually the complete opposite on my trainer road workouts.  below is a typical trainer workout for me

Lots to read to answer your questions in here:

http://support.trainerroad.com/entries/20655098-Power-vs-VirtualPower-vs-Heart-Rate-Which-one-is-right-for-me-

How was your LTHR measured?  The 20 minute test?

Also remember that HR is a lagging indicator and you will pretty much always experience cardiac drift on a trainer, so I'm not surprised your HR may be a little more in a higher zone than your wattage.  And TR is only a fair estimate of power anyway.

And the "sweet spot" definition is only two beats wide in TR...which I find a little weird.

I've been trying to do more Z2 riding lately, and that Guyat workout creeps me up into low Z4 during the final interval (so I won't use it for a Z2 ride).

Use power.

The problem with HR is that the fitter you get, the LOWER your HR goes for a particular power level. It becomes very pronounced when you significantly improve. I just got a powermeter, and if you looked at my HR despite increasing power, it looks like I'm getting worse or slacking off, whereas my TSS & FTP have both gone up progressively.

Don't worry about being in 'higher' HR zones based on the virtualpower training - if you're working off your power test results, you're exactly where you need to be, even if the HR seems high.

I suspect your HR zones are too low or your FTP setting is too high. It's not normal that you ended up above your threshold HR zones 4 minutes into a threshold interval.

How you determined each set may give a hint of which one may be better.

And there is nothing wrong with looking at HR and power. Choose which one will guide you, but be aware of the other and use it as a backup.

How hard was that workout for you ?

Im the same way.  Here is my ride from this morning. http://www.trainerroad.com/cycling/rides/272531   Im guessing my HR zones are lower because I only did the 8 minute test.  I have another test on Monday(my third) and plan to use the 20 minute test.  My In my first two tests my LTHR stayed the same wile my FTP went from 217 to 247.  I expect the LTHR will change with this next test.