Training zones

Lactate thresholds: LT1, LT2 and lactate testing

Lactate thresholds are the two turn points in blood lactate as effort rises. LT1 is where lactate first rises above its resting level, roughly the top of truly easy effort. LT2 is where lactate starts building faster than your body can clear it, close to your threshold.

A lactate test finds your heart rate, pace and power at each turn point, which makes it one of the most personal ways to set training zones.

By Nick Buckley-Mills, Team GBR age-group triathlete Reviewed 15 October 2026 6 min read

At a glance

Turn point	What happens	Roughly where it sits
LT1also aerobic threshold	Lactate first rises above your resting baseline	The top of easy, Endurance effort
LT2also anaerobic threshold	Lactate starts to climb steeply and no longer settles	Close to FTP, threshold pace or CSS

What is lactate?

Lactate is a fuel your body makes and uses all the time, not a waste product. Muscles produce it even at rest, and other muscles, the heart and the liver take it up and use it. As effort rises, production climbs. At low intensity your body clears it as fast as it appears; past a certain point it can't keep up, and the level in your blood rises.

That changing balance is what a lactate test measures. The level in your blood isn't the cause of fatigue, but it is a useful window into how hard your metabolism is working.

How does a lactate test work?

You exercise at steadily rising intensities, and a drop of blood is taken from a fingertip or earlobe at the end of each step. Each stage usually lasts a few minutes, on a bike, a treadmill or sometimes in the pool. Plotting lactate against power, pace or heart rate draws a curve that is flat at first, then bends upward.

Where exactly you mark LT1 and LT2 on that curve depends on the method. A 2009 review counted 25 different lactate threshold concepts, from fixed values to the first rise above baseline to the steepest bend in the curve. Two labs can test you on the same day and report different numbers, so always ask which method was used.

How do lactate thresholds become zones?

Your heart rate, pace and power at LT1 and LT2 become the edges of your zones, instead of a percentage of a test result. Effort below LT1 is easy, between LT1 and LT2 is moderate, and above LT2 is hard. That is the three-zone model most research uses. In a five-zone model, LT1 sits around the top of Zone 2 (Endurance) and LT2 around Zone 4 (Threshold).

The advantage is that the zones come from your physiology, not from a formula that assumes your thresholds sit where an average athlete's do. See how training zones work for the five-zone model LOAD uses.

What is lactate-guided training?

Some athletes now carry portable lactate meters and check their blood during sessions, to keep lactate inside a set range. The best-known example is the Norwegian approach to threshold training, where intervals are paced to hold lactate in a band below LT2, rather than to hit a power or pace target.

This turns lactate into a live, internal measure, like heart rate: it tells you what the session is doing inside your body. Power and pace are external measures: they tell you what you are producing. The two can disagree. On a tired day, the same power can push lactate higher, and lactate-guided training would slow you down where a power target wouldn't.

What changes for triathletes?

Your turn points are different in each sport, so one test doesn't cover all three. Heart rate at LT1 and LT2 is usually lower on the bike than on the run, and power or pace at each point is sport-specific by definition.

Bike and run tests are the most common. Swim lactate testing is possible poolside but much rarer, so most triathletes set swim zones from CSS.
LT1 matters a lot for long course. Full-distance pacing often sits close to LT1, and half-distance not far above it, so knowing where LT1 is can be worth more than knowing LT2.
Test fresh and in the same conditions. Testing after a hard training block, or on a different diet, will move the curve.

Should you trust a lactate test?

Trust it more than a formula. Don't trust it as a fixed truth. A lactate test is the most personal way most athletes can set their zones, and it still describes a moving target.

What it gets right

It is personal. It finds where your turn points actually are, instead of assuming they sit at a standard percentage of threshold.
It finds LT1. Field tests like FTP only estimate the top end. A lactate test also shows where easy really stops, which is often the boundary athletes get wrong most.
It measures the inside. Like heart rate, lactate reflects what the effort costs you, not just what you produce.

Where it falls short

Turn points are a progression, not a line. The lactate curve bends gradually. LT1 and LT2 are points chosen on a smooth curve, and the body doesn't switch state as you cross them.
The method changes the answer. With dozens of ways to define a threshold, the "same" test can produce different numbers.
It's a snapshot. Fatigue, carbohydrate intake, heat, hydration and the length of each stage all shift the curve. Your thresholds also move as you get fitter or lose fitness, so last spring's test may not describe you today.
The measurement can be wrong. Sweat on the skin, a poor sample or a different analyser can change a reading.
It can become another number to chase. Holding lactate in a band is precise, but precision isn't the same as the right session for you.

How to use it anyway

Treat your turn points as ranges, not lines. Retest under the same conditions and protocol so results are comparable, and check them against how sessions feel and how your heart rate behaves. If the numbers and the feel disagree for weeks, it is probably time to test again.

Common questions

Do I need a lactate test to train well?

No. Field tests such as FTP, threshold pace and CSS set useful zones for most athletes. A lactate test adds a personal picture of both turn points, especially LT1, which field tests only estimate.

Is LT2 the same as FTP?

They are close but not identical. FTP is a performance test of roughly an hour's sustainable power; LT2 is a turn point on your lactate curve. For many athletes they land near each other, but the exact numbers depend on the test and the method.

Why do different labs give different thresholds?

There are dozens of ways to define a lactate threshold, from fixed lactate values to the first rise above baseline to the steepest bend in the curve. Protocol, stage length and the analyser also matter, so compare results only when the method is the same.

How often should I retest my lactate thresholds?

Turn points move with fitness, so many athletes retest every few months or at the start of a new training phase, under the same conditions and protocol so results are comparable.

Sources

Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Medicine. 2009;39(6):469–490.
Jamnick NA, Pettitt RW, Granata C, Pyne DB, Bishop DJ. An examination and critique of current methods to determine exercise intensity. Sports Medicine. 2020;50:1729–1756.
Brooks GA. The science and translation of lactate shuttle theory. Cell Metabolism. 2018;27(4):757–785.
Casado A, Foster C, Bakken M, Tjelta LI. Does lactate-guided threshold interval training within a high-volume low-intensity approach represent the "next step" in the evolution of distance running training? International Journal of Environmental Research and Public Health. 2023;20(5):3782.
Impellizzeri FM, Marcora SM, Coutts AJ. Internal and external training load: 15 years on. International Journal of Sports Physiology and Performance. 2019;14(2):270–273.