What is LT1, and how do you measure it without a lab?
Many runners know one threshold: the point where it gets hard. There is a second one, lower down, and it decides how easy your easy days really are.
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 upper of two turn points — LT2 in the literature — and it marks where steady becomes hard.
The lower one gets less attention, even though it matters more for how you build a week. That is the aerobic threshold, LT1 in the literature: the upper limit of your genuinely easy endurance work, the highest pace at which your body still clears lactate without effort.
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. The familiar guideline that most of your training time should be easy 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, and 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. The drawbacks: it costs money, you do it at most a few times a year, and the result holds for that 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 and compare the ratio between pace and heart rate in the first and the second half. If that ratio stays roughly the same — a common rule of thumb is less than five percent of drift — you were below your aerobic threshold. If your heart rate walks away in the second half at the same pace, you were above it. Repeat that 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: the point where your heart rate stays flat during steady work. That is precisely what software can do — provided it looks 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 estimate comes from sessions in which your heart rate stayed flat during steady work.
Three things keep that honest. There is no estimate without enough easy sessions: if there are too few, the app says it does not know yet rather than inventing something. The result is bounded against your second threshold, so a single outlier cannot upend your zones. And you get a proposal, not a silent change: you see what the number rests on ("estimated from eleven easy endurance runs over recent months") and you either take it or leave it.
Once your LT1 is set, it becomes the ceiling of your zone 2. "Easy" then ends at your own turn point instead of at a fixed fraction of your threshold heart rate.
Frequently asked questions
Can I determine LT1 with a heart rate strap?
The drift method gets you a long way. You do not need a lactate meter, but you do need a steady endurance session of at least 45 to 60 minutes, and the willingness to repeat it a few times at slightly different paces.
Does LT1 change with training?
Yes. It is one of the first things to move as you build aerobic volume: you get faster at the same heart rate. That is why a number from last season says little, and why measuring continuously beats one test a year.
Is LT1 the same as zone 2?
Nearly. Zone 2 is a division within a model; LT1 is a physiological turn point. In a well-built zone model the top of zone 2 coincides with LT1 — but in models that use only percentages of your maximum heart rate, zone 2 can sit some way off.
I do not have a heart rate monitor. Can I still do anything?
Yes: the talk test, and your pace. If you know what pace you can hold for an hour in a controlled effort, your easy endurance work sits clearly below it. Cruder than measuring, but more useful than running the same middling pace every day.
Trimetra estimates your aerobic threshold from your own sessions and puts it forward as a proposal, separately per sport. You can try everything for fourteen days; recording and collecting stay free after that.