Aerobic threshold (LT1)

What is the aerobic threshold?

Your body is always producing lactate and always clearing it again. At low intensity the clearing keeps up without effort, and the value in your blood stays close to your resting level. The aerobic threshold is the point where that value first rises clearly above rest: the first of two turn points, called LT1 in the literature. The second, LT2, is the threshold LTHR and FTP belong to.

The practical value lies in the range below it. Below LT1 you can build volume without it eating your recovery; above it begins the range that is too steady for recovery and 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.

How do you determine your aerobic threshold?

The most accurate route is a lactate test in an exercise lab: a protocol of rising load, where the first clear rise above the resting value marks LT1. The drawback is that the result holds for that day and that shape; after a training block the number already fits less well.

Without a lab, two methods get you a long way. The talk test: at or just below LT1 you can still speak in full sentences, above it your speech turns halting. And 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 you were below it; if your heart rate walks away at the same pace you were above it.

Determine it per sport. On the bike both LT1 and LT2 usually sit lower than when running, because less muscle mass is active and your weight is carried. And retest periodically: LT1 is one of the first things to move as you build aerobic volume.

How Trimetra personalises this

Trimetra stores a separate running LT1 and cycling LT1. You can fill it in yourself, or leave it empty: the app then estimates it from your own easy endurance sessions — the ones in which your heart rate stayed flat during steady work — and puts it forward as a proposal, with the source line alongside. Without enough easy sessions there is no estimate; the app says it does not know yet. Once an LT1 is set, it becomes the ceiling of your zone 2, so "easy" ends at your own turn point instead of at a fixed fraction of your threshold heart rate.

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