Does this lactate threshold calculation by Garmin make any sense?

Notne what the post above means is that if you are running above your lactate threshold the lactic acid in the blood/muscles will rapidly increase - hughly trained athletes have both a higher threshold generally and a higher tolerance to lactic acid buildup. In practical terms if you run for any length of time over that threshold you will quickly experience a high level of pain in the muscles and at a certain point the muscles will stop working until the lactic acid is metabolised and removed.

In short I think for you the LTHR is wrong - have a look at the Joe Friel article on Training Peaks https://www.trainingpeaks.com/blog/joe-friel-s-quick-guide-to-setting-zones/

He suggests a 30 min run at race pace - marking a lap after 10 mins and taking average HR for the last 20 mins as your LTHR - I suspect that will give you quite a different figure from the Garmin - at the moment mine ob the 5+ shows as 154 BPM 9:02 but my most recent Easy run was done at a pace of 8:58 overall (including warmup/down) with an average HR of 145 so not sure how the 5+ calculates but I'm sure it's not correct for me

When the LTHR first came out on Garmin devices (late 2015 - I was using a Fenix 3 at the time), there were some Garmin info videos along the lines that the LTHR and T Pace were the HR and pace that would see you complete a 10km run at that constant pace/HR without ‘creaming in’; you could go faster, but your pace would start to drop off and HR rise before reaching 10km. 10km is 6.21 miles and you are running 4.5 miles; so it’s entirely possible you can run that shorter distance faster than T Pace without hitting the point your HR rapidly rises or pace falls away.

My other long term observation is that the auto detect LTHR and T Pace tends to be a lower HR and slower pace than if I do the LTHR Guided Test. This seems to be because I do the guided test on a purposefully flat run, but when it auto detects iduring a normal run, I can see the only place my HR has gone above LTHR is on gentle uphill inclines (so Pace is slower and HR higher than on the flat). I think the LTHR algorithm has a grade tolerance that gets rid of steep hills, but gentle uphill gradients are a problem in my experience.

Thanks guys, those were massively helpful posts.

JSRUNNER, I was thinking about going out and trying that "last twenty minute average" described in the article you linked to, but was wondering about a detail - at the end of every run for the last few hundred meters I try to kick it in hard, winding up in a hard sprint at the end. The link didn't say to run the last twenty yards at constant pace or otherwise ... do you have a sense of the right way to do that?

Thanks!

The right way to do that effort is to run as hard as you can at a pace you can maintain for 20 mins. If you do it right, there will not be anything in the tank left to sprint with. You might be able to pick up the pace, but sprinting should not be possible. It can take some time to get this right. One thing you want to avoid is going out too hard and dropping off the pace. You need to maintain as close to a constant pace as possible for that full 20 mins...but hard.

Thanks, philipshambrook. I am throwing the word "sprinting" around too loosely ... I should have said something like "running at the same perceived effort as a sprint". I happened to see my shadow during that "sprint" recently, and it was definitely not looking like any kind of real sprint!

Notne - philipshambrook is spot on - let us know how you get on - I'm still struggling myself to get a reasonably credible LTHR from the watch's own calculation

"running at the same perceived effort as a sprint".

Now being a little pedantic. A sprint is an effort that can hold only for a few seconds; by definition it's maximal, all-out effort. Whereas what you need to be aiming for is an effort somewhere less than that. Depending on your level of fitness it'll likely be a pace that you can hold for 3 to 5 kms.