What is LT1, and how do you measure it without a lab?

Two thresholds, two different questions

If you train by heart rate or power, you probably know your threshold. Your LTHR, your FTP, your hour pace: that is the point where steady becomes hard. Such a point is called a turn point: below it and above it, your body handles the effort differently. There are two of them. This is the upper one, and in the literature it is called LT2.

The lower one gets less attention, even though it matters more for how you build a week. That is the aerobic threshold. In the literature it is called LT1. It sits at the highest pace at which your body still clears the lactate you produce without effort. That is also the upper limit of your genuinely easy endurance work.

LT1 answers the question: up to where is easy really easy? Below that boundary you can build volume without it eating your recovery. LT2 answers a different one: where does it get genuinely hard? That is your LTHR or FTP, and it drives your intervals and your race pace.

Between the two sits a large area. That is where much of what people call "just running" lives. A bit too steady for recovery, a bit too light for a real stimulus. You probably know the rule of thumb that most of your training time should be easy. It is often quoted as eighty percent easy against twenty percent hard. That eighty percent refers to the space below LT1, not to everything below LT2. Mix the two up and you train your easy days too hard as a matter of routine. A single session will not show it. It stacks up over weeks.

Roughly where LT1 sits

LT1 sits clearly below LT2, but how far below differs from person to person. A percentage of your maximum heart rate, or of your threshold, is therefore a starting point rather than an answer.

Only your own sessions can say where it sits for you. And that number is not a constant either: it moves with your form.

How a lab measures it

In an exercise lab you work through a protocol of rising load, with blood samples taken at fixed intervals. The first clear rise in lactate above the resting value marks LT1; the second, steeper bend marks LT2.

That is accurate, and with a physiologist alongside it is well interpreted too. There are three drawbacks. It costs money. You do it at most a few times a year. And the result holds for that one day, on that treadmill, in that shape. Six weeks and a training block later you have a different number.

How to approach LT1 without a lab

The talk test. At or just below LT1 you can still speak in full sentences. Go above it and it turns halting: short sentences, snatching a breath in between. Crude, but usable — and free.

The drift method. Do an hour at a steady, easy pace. Afterwards, look at how many beats per minute that pace cost you: first in the first half, then in the second. If that stays roughly the same, you were below your aerobic threshold. If your heart rate walks away in the second half while your pace stays the same, you were above it. A common rule of thumb puts the boundary at five percent of drift. Repeat it a few times at slightly different paces and you circle in on your boundary.

Your own history. If you have done enough easy sessions, the answer is already in your data. It is the point where your heart rate stays flat during steady work. Software can work that out for you. But then it has to look at your sessions and not at an average of everyone.

What Trimetra does with it

Trimetra estimates your LT1 from your own easy endurance sessions, separately per sport. Running and cycling each have their own anchor. The first question is which sessions were easy enough to count.

That can show in two ways. The first is your own judgement: if you gave the session a low perceived effort afterwards, it counts. The second is the measurement itself, for when you filled nothing in, or found it harder afterwards than it really was. Two things then have to hold at once. Your pace was well below your own threshold pace — on the bike: your power well below your FTP. And your heart rate stayed with it for the whole session. The estimate comes from the heart rates of the sessions that remain.

Three things keep that honest. The first is the bar for evidence. Without enough easy sessions there is no estimate; the app then says it does not know yet, rather than inventing something.

The second is a boundary. Your aerobic threshold sits well below your second threshold by definition, and the estimate is not allowed to fall outside that. So a single outlier cannot upend your zones.

The third is that you decide. You get a proposal, not a silent change. You see what the number rests on: "estimated from eleven easy endurance runs over recent months". After that you take it, or you leave it.

If you take the proposal, something changes in your zones. The top of zone 2 moves to your LT1. Without that value, the app learns the top from your own easy sessions; only when there is too little evidence for that does it fall back on a fixed percentage of your threshold heart rate that is the same for everyone. With it, the top sits at your own turn point.

Frequently asked questions

Trimetra estimates your aerobic threshold from your own sessions and puts it forward as a proposal, separately per sport. The source line comes with it, so you can see what the number rests on before you take it over.