so i have a segment thats 34 minute long with an average HR of 187. does this mean my FTP will be gauged closer to 190bpm (approximation)? say i dont have a power meter and i want to do a 40k TT, should i pace my HR at 190 bpm the entire way? thanks

If you are going as hard as you can sustain for the 34 minute segment, you should take your average HR for the last 20 minutes of that segment and use that as a good approximation of LTHR.

Then try to time-trial at your LTHR. You might go slightly over on any uphill sections, and be slightly under on the downhills, but try to stay there or thereabouts until your final push for the line, when you max out.

thanks chasm54!

I don't know about you...but my heart rate is all over the map depending on my level of hydration, fatigue, heat/cold stress etc...

I would find it very difficult to pace a TT based on heart rate.

My view would be that if I was dehydrated, fatigued or not warmed up I shouldn't be starting the TT in the first place.

Pacing is difficult. But if one does not have a powermeter, HR and perceived exertion is all one has. And as far as I am concerned, HR is reasonably predictable. My LTHR tests at 162. When I was racing (last year, I haven't done any this season) my average HR in crits lasting around an hour was - 162. That's pretty decent evidence that the numbers are consistent and predictable.