Blood Lactate Testing: What It Is, Why It Matters, and What’s Really Happening in Your Body

Felicia Sciortino
Jan 22
4 min read

Blood lactate testing has become more common in endurance sports, but it’s still often misunderstood. For many athletes, lactate is associated with burning legs or fatigue. In reality, lactate is not the problem—it’s a powerful marker that tells us how your body is producing and using energy at different exercise intensities.

When used correctly, blood lactate testing helps take the guesswork out of training and pacing by showing exactly how your body responds as intensity increases.

What Is Blood Lactate Testing?

Blood lactate testing involves taking small blood samples (usually from a finger or ear) during progressively harder stages of exercise. Lactate concentration is measured in millimoles per liter (mmol/L).

As exercise intensity increases, lactate levels rise—but the pattern of that rise is what matters most. These changes help identify key physiological thresholds that guide training zones and race strategy.

What Is Lactate, Really?

Lactate is a normal byproduct of carbohydrate metabolism and an important fuel source. When your muscles break down glucose for energy, lactate is produced—especially as intensity increases.

Importantly:

Lactate can be shuttled to other muscle fibers and organs for energy
The heart prefers lactate as a fuel source
The liver can convert lactate back into glucose

Fatigue isn’t caused by lactate itself, but by the accompanying changes in acidity and metabolic stress when lactate production exceeds clearance.

LT1: The First Lactate Threshold (Aerobic Threshold)

LT1, often called the aerobic threshold, is the first noticeable rise in blood lactate above baseline levels.

At intensities below LT1:

Lactate production and clearance are balanced
Blood lactate remains relatively stable (often ~1–2 mmol/L, though this varies by individual)
Fat oxidation is high
Effort feels comfortable and sustainable for long durations

This zone corresponds to:

Easy runs and rides
Long aerobic sessions
Recovery workouts

Training below and around LT1 builds your aerobic base by improving mitochondrial density, capillary development, and your ability to use fat as fuel. This is why a large percentage of endurance training should live here—even for highly trained athletes.

What Happens at LT1?

As you cross LT1:

Carbohydrate use begins to increase
Lactate production starts to slightly exceed clearance
Breathing rate increases subtly
Effort is still controlled, but no longer “effortless”

Blood lactate rises gradually, not sharply. This is an intensity you can hold for hours, but it’s no longer ideal for recovery.

LT2: The Second Lactate Threshold (Anaerobic Threshold)

LT2, often referred to as the anaerobic threshold, represents a much more significant shift in physiology.

At LT2:

Lactate production greatly exceeds clearance
Blood lactate rises rapidly (often >4 mmol/L, though this is individual)
Reliance on carbohydrate metabolism increases
Acidity increases in the muscle
Fatigue accumulates quickly

This intensity corresponds to:

Tempo efforts
Threshold intervals
Race pace for events lasting ~30–60 minutes

Efforts at or above LT2 are not sustainable for long periods and require careful dosing in training.

What Happens Between LT1 and LT2?

The space between LT1 and LT2 is a critical training zone. In this range:

Lactate rises steadily but remains manageable
The body is working harder but still in control
You’re training your ability to tolerate and clear lactate

Well-trained endurance athletes typically have:

A higher LT1 (they can go faster before lactate rises)
A higher LT2 (they can sustain faster speeds before fatigue sets in)
A wider gap between LT1 and LT2

This is a sign of strong aerobic fitness and efficient energy use.

Why LT1 and LT2 Matter for Training

Understanding both thresholds helps refine training far more precisely than pace or heart rate alone.

LT1 helps define true easy and aerobic training zones, preventing “gray zone” training
LT2 helps guide tempo work, race pacing, and high-intensity intervals

Blood lactate testing allows these thresholds to be identified based on your physiology, rather than estimated from formulas or assumptions.

Lactate Testing vs. Guessing Your Zones

Heart rate and pace are valuable tools, but they’re influenced by fatigue, heat, stress, hydration, and sleep. Lactate testing provides objective insight into how your body is responding internally.

When lactate data is paired with heart rate, pace, and perceived effort, it creates a powerful framework for intentional, efficient training.

Who Benefits Most From Lactate Testing?

Blood lactate testing is especially helpful for:

Runners and triathletes building aerobic fitness
Athletes plateauing despite consistent training
Athletes returning from injury who need objective boundaries
Those preparing for key races

Testing doesn’t need to be frequent—periodic testing can help ensure training is aligned with your current fitness and goals.

The Takeaway

LT1 and LT2 aren’t just abstract physiology terms—they represent real, trainable transitions in how your body produces and uses energy. Blood lactate testing helps identify these points so training can be structured with intention rather than guesswork.

When you understand what’s happening beneath the surface, you can train smarter, pace more confidently, and get more out of the work you’re already doing.