Trying to understand Garmin's LTHR

Garmin has 4 ways of setting HR zones - BPM, %Max HR, %HRR and %LTHR. You have to set your zones to be based on %LTHR.

My zones are set based on LTHR and I reset them whenever my LTHR changes. 

I spent the last 80 minutes of my marathon with my HR above 173 which is my current Garmin LTHR. 

Then I would suggest turning off automatic detection of max HR and set your max HR manually to 197 or higher. You can do your own max HR test. Then redo the guided LTHR test on the Garmin. If it still gives you 173, then you might want to adjust your zone % accordingly.

First of all: forgot about the maxHR detection from Garmin. If you hit a higher number than Garmin has detected automatically, then the higher value is (at least) your maxHR.

HR zones are just a simplified concept. My average HR in my last marathon was 177 (89% maxHR). I also set my zones based on LTHR, and 67% of the race I was on Z5

I'm about to do an physical Lactate test and add my zones manually as well. What else should I change to make proper use of it and make sure Garmin doesn't override it?

I don’t think Garmin ever changes your zones on their own. They have automatic detection of max HR and LTHR but even then you need to go in and « reset » your zones whenever those values change. 

Once you set your zones they should stay that way. (At least that’s been my experience.)

jambongris said:

I've also heard it referred to as your one hour pace.

There is a lot of talk in endurance sports about the maximum steady state which is understandable since we are trying to maximize our capacity to sustain an effort for a certain duration.

Different things that are often considered the same. You have three ways to look at the "anaerobic threshold". Each way is valid, but technically corresponds to different physiological aspects.

First, there is a blood chemistry aspect, with the concentration of lactate. As soon as you exceed the first threshold, lactate is produced but is recycled almost as fast. Until the anaerobic lactate threshold, the concentration increases slightly. At the second treshold, it increase much faster because the rate of production exceeds more and more the rate of recycling. As a result, there is an accumulation of lactic acid in the muscles which creates the muscular sting that prevents you from going overboard. Unfortunately, LT2 is also called Maximal Lactate Steady State, which typically underestimates the actual maximal metabolic steady state (The maximal metabolic steady state: redefining the 'gold standard' - PubMed (nih.gov). This is one reason why endurance athlete can run "long distances" close or above LT2.

The only duration notion is that in theory, exactly at LT2, there is a steady state where you could go on "indefinitely". In practice, we are looking at about an hour. But other phenomena will push the heart rate higher than it was at LT2.

Second, there is a ventilation aspect. At the first ventilatory threshold, typically 75%-80% of HR Max, the respiration starts accelerating. At VT2 the rate of respiration makes it hard to have a conversation, and the rate of VCO2/VO2 starts growing much faster to converge at around 1 plus change at VO2 Max. Garmin uses HR and HRV to estimate VT2, but again, the heart rate will not stay at this value for long (see below).

Finally, you have an output aspect of LT (pace or power). LT2/VT2 is assimilated to FTP (technical defined as a duration notion) or CP which is defined as a steady state notion. Therefore FTP and CP are again different but represent a theoretical sustainable steady state. Top marathonians complete their race at about 95% of their CP. Generally CP is higher (maybe around 5-10%), so there is a good chance that their HR will not stay in the threshold zone either.

Finally, on top of all this "not really steady state" aspects, there is a heart drift aspect in longer races (above 1:30mn maybe). As the body loses fluids, it needs to increase the respiration rate so that the volume of blood pumped is enough to bring the required oxygen.

jambongris said:

How is the threshold between Z4 and Z5 my one hour pace

The LTHR is not linked to your 1h pace. As you start running at maximum steady pace for an hour, your heart rate will increase, cross the LT1, then the LT2 and venture into Z5. It will do so for a 5k as expected, but also for a 10k, HM. For HM and marathons, the cardiac drift will add to the equation. That said, we can associate a pace at LT2, which in theory would be the pace you could sustain indefinitely. It turns out this is rather the pace you can sustain for a 10k, or longer, as described above, if you are an elite marathonian.

jambongris said:

I've briefly hit an HR of 197 at the tail end of my last two races according to my HRM Pro+ but Garmin estimates my MaxHR at 194

This is a good sign that your HR Max is higher than what your watch is saying. The algorithm looks for qualifying peak HR periods, but is still subject to errors due to HR wrist or strap limitations. I would take your peak HR for a 5k and 10k, add 5bpm and go with it.

jambongris said:

Garmin also estimates my LTHR at 173 currently which mostly lines up with their definition above.

The ration of LTHR to HRMax is a decent way to verify HR Max, but choosing the right % is not easy. It could be as low at 80% for untrained athletes, and in the 95% range for trained athletes. An average of 90%-92% is good average to use, but your mileage will vary.

jambongris said:

Are my zones set up incorrectly or am I just misunderstanding LTHR?

If you use any of the default HR zones system (HR Max, LTHR or HRR) on a Garmin watch, it should be plenty good enough for training purposes. As you know though, HR is laggy and it drifts, so using pace (on flats) or power is better to train with steady effort intervals.

After all, the purpose of all this endurance training is to increase VO2 Max and equally importantly, the ratio of VO2 LT1 and VO2 LT2 to VO2 Max. The more you can use your VO2 Max capacity while keeping in check lactate accumulation, hydration and carbs consumption, the better.

In fact, CP has proven to be a better predictor of racing performance and VO2 Max, although you need a big VO2 Max to be able to sustain 2+ hours at 95% of CP, for sure.

Etupes25, thanks for detailed responses. There’s a lot to digest there.