I recently started with 80/20 to train for my first triathlon (olympic distance). I’m on about week 7 of the plan and just last week had a Cycling Tempo workout that I used to reestablish my HR zones (don’t currently have a power meter yet, so using HR for my bike training). During that tempo workout my avg HR during the 15 minutes was about 172 so I set that as my threshold and recalculated my zones. The following workout was a cruise interval with 5mins hard in zone 3 (162-171) and I found it difficult to maintain my HR in that zone. In the 3 intervals I maintained an avg of 155, 150 and 162 respectively.
I know HR is an unreliable metric due to the conditions of each session (temperature, etc). The day that I did the tempo workout it was a hot afternoon ride while the following cruise interval was an early morning ride. Is my difficulty with the cruise interval workout due to incorrectly testing for my threshold? Or is this just an adjustment period to the new HR zones? Should I stick with it or try to retest for new HR zones?
It’s really a matter of the nature of HR. HR is sensitive the environment, particularly temperature. For every degree over 70 F your bpm will increase by about 1. Therefore, your experience is consistent with that phenomenon. You tested in environment A but are training in environment B. (HR is also 4 beats higher in the PM than AM for the same intensity. Indoor training is about 5 beats lower than outdoor, if you ate in the last hour HR is 5 beats higher, if you are stressed HR is higher…)
This is also evident from your own data: You can maintain 172 for 15 minutes, but struggle with it for 5 minutes just a coupe of days later. The difference? The environment.
I think your LTHR is indeed 172, but it’s 172 when training in the conditions under which you tested your LTHR.
Sometimes, it’s not HR, it’s just fatigue. Even if you switched to a power meter, there will be days when you can’t hold Zone 3 power if you are thrashed.
The workarounds are not only a power meter, but also just giving yourself permission to use perceived effort on some days.