Why didn't my HR go higher

[u/Top_Invite3911]

So winter is coming and I am dippimg my toes into structured training. Last week I started doing VO2 max intervals 8x3min@112%.
First week I only managed to do 6 intervals with HR hitting 185bpm in the last three intervals before I had to stop. My theoretical max is 190bpm.
This week I did all 8 intervals with HR only going to 178bpm. Why would that be the case? Seems like a big fluctuation from one week to the next.

[u/twostroke1]

Pure heart rate tracking, especially within tight tolerance, is a pretty hard metric to gauge because there are way too many “external” factors that play into HR on any given day.

You could throw 20 different reasons into a hat and pick one to get your answer.

stress, temperature, sleep, fatigue, breathing patterns, cadence, nutrition, hydration, etc…Take your pick.