Too much time in Z5

I would manually raise the max HR significantly.

For correct HR zones you better use a HR chest strap, I bought a polar H10, works flawless, than set HR zones based on lactate threshold so after every run with the chest astrap if the watch find a new threshold value (good or bad) You'll be notified and the HR zones will adjust automatically trough time.

PS: in my last 10k PB attempt I spent 86% of the time in Z5, 40min out of 45, so it's possible to be in Z5 for long time, consider that's a range, low Z5 is near LT, the pace you can keep for a fast 10k. If You were talking about the nonsense of being in z5 while running a low pace training you are correct, the fault is probably related to wrong HR zones. Best solution is chest strap and settings change, be sure you have the option of new hr max recording tuned on if you don't have a chest strap.

If you change HRmax the zones are not adjusted automatically. To me it seems to be an annoying garmin bug. If you has a z5 for example, set to 165-187 and real max HR will be set to 210 automatically later, your zones remains divided wrong. You must do an manual zones reset after every auto adjustment maxHR/LTHR.

Thanks all for your remarks.

I should indeed switch to chest strap at some point.

For now, I did an hr max test (based on a suggestion found online) and as expected my hr max was auto increased to 195. (max recorder during the session was 197).

@global_local I went to "reset default zone", is it what you suggested? weird that is does not adjuste automatically...

Is it now possible the recompute the stats on all previous sessions with this new hr max value?

If you did 40 minutes in Z5, your zones are also not correct. It should by definition not be possible. For most people, 3-5 minutes should be doable, and for top athletes something like 5-10 minutes. I ran a personal best 10K (just under 44 minutes at age 52) a couple of months ago, and I did not touch Z5 except for the last few seconds.

You are probably talking about values around max hr or high Z5, than yes, I could not last long at that pace, but a 5km or 10km race can be run with a high % of Z5, by definition, because you are running a short distance at high pace. If the lactate threshold is the balance point where the amount of lactate produced equal the amount you can dismantle, and that is the upper limit of Z4, the edge between Z4 and Z5, it's reasonable that just 1bpm higher than your LT is already Z5, and if you can handle the LT by definition for a 10k that means that around 1bpm more is already Z5 and you can handle it slightly less than your 10k pb. Of course I'm talking about  low Z5, my Z5 is 174-196bpm, I can handle 174 bpm for almost 10km but I can't handle 185 or more that much, both values are Z5 but ther's a big difference.

I was referring to 5 zones based on LTHR in Garmin connect, I train with a chest strap and using zones based on LTHR is considered more accurate because max hr decrease trough time as you age, LT instead can grow as you improve and is more dynamic (can also decrease if untrained), max hr has less variability and I've read a lot about which is the best method of choosing HR zones and LTHR is considered the best, the top tri athletes from Norway use a lot LT measurements, multiple times a day, they take samples of blood, the good stuff with all the top tier tech... of course.

Probably depends on you and what are your goals, I'm not pretending to know everything, but at least for me and for what I know LTHR is a good method for setting HR zones.

Yes, all the top athletes are training lactate threshold based now, doing double interval sessions in one day. I am also using a chest strap, but I don't always trust the lactate threshold measured by the watch, and sometimes it can be a long time between each valid measurement. One bad measurement will mess up the zones. That is why I prefer the stability of fixed zones based on max HR. My goals are just to stay generally fit, and since I am an engineer, I am fascinated by measurements, data logging, analysis and technology.