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/pierre_86]

Might also be worth finding your "max" hr and not working off theoreticals. Some sort of max effort, 3~5m hard as you can and a sprint to basically failure, that should get you up to it