What is Lactate Threshold?

What is Lactate Threshold?

Lactate threshold is the exercise intensity where blood lactate stops sitting at a low, stable resting level and starts climbing sharply because your muscles produce it faster than the rest of the body can clear it. At rest and during easy effort, blood lactate stays around 0.8 to 1.5 mmol per litre. As you speed up, more of your energy comes from breaking down carbohydrate, and lactate production rises.

Below threshold, other tissues, mostly the heart, slow-twitch muscle fibres, and the liver, mop up that lactate as fast as it appears, so the blood concentration barely moves. Cross the threshold and the balance tips: production outruns clearance, lactate pools in the blood, and the effort that felt controlled now feels progressively harder and time-limited.

Physiologists actually describe two turning points. The first, LT1 or the aerobic threshold, is where lactate first creeps measurably above baseline, roughly 2 mmol per litre, and it sits near the top of Zone 2. The second, LT2, is the highest intensity you can hold while lactate still plateaus rather than spiralling upward, often called the maximal lactate steady state.

LT2 is the more famous number because the pace you can sustain there predicts endurance race performance better than VO2 max does. Two runners with identical VO2 max can differ by minutes over 10 km simply because one holds a higher fraction of that ceiling before lactate runs away.

How it works

Lactate threshold reflects the tug-of-war between how fast your muscles make lactate and how fast your body clears and reuses it. When you exercise, glucose is broken down through glycolysis into pyruvate. If oxygen delivery and mitochondrial capacity keep up, pyruvate is oxidised aerobically and little lactate forms. As intensity climbs, fast-twitch fibres recruit and glycolysis accelerates past what the mitochondria can immediately burn, so the enzyme lactate dehydrogenase converts the surplus pyruvate and NADH into lactate and NAD+.

That reaction is not a failure, it is the trick that regenerates NAD+ so glycolysis can keep producing energy quickly. The lactate itself is not waste. Through what George Brooks named the lactate shuttle, it travels out of the working fibre and is taken up by nearby slow-twitch fibres, the heart, and the liver, where it is either oxidised back to pyruvate for fuel or rebuilt into glucose via the Cori cycle.

Below threshold, this shuttle keeps blood lactate flat. At threshold, recruitment of glycolytic fibres and the sheer rate of production overwhelm the clearance machinery, and blood lactate rises steeply. Training raises the threshold by expanding the clearance and oxidation side of the equation: more mitochondria, denser capillaries, and more lactate-transport proteins (MCTs) let you produce a given amount of lactate and still clear it, so you can hold a faster pace before the curve bends upward.

The scale

There is no single equation for lactate threshold; it is read off an incremental step test as the intensity where blood lactate rises sharply. Practitioners split training into zones around the two thresholds. A common three-zone model:

How to apply it

• Incremental lab step test: The gold standard. You ride or run at rising stages (for example +25 watts or +0.5 km/h every 3 minutes) while a technician takes a fingertip or earlobe blood drop each stage, then plots lactate against intensity to find the breakpoints.
• Fixed 4 mmol/L (OBLA): A simple rule that calls the threshold the intensity producing 4 mmol/L of blood lactate. Easy to compare across people, but it can overestimate or underestimate any one athlete's true steady state, which may sit anywhere from 2 to 8 mmol/L.
• Maximal lactate steady state test: Several constant-pace efforts on separate days find the fastest pace at which lactate rises less than 1 mmol/L over the final 20 minutes. It is the most valid measure of LT2 but demanding, since it needs multiple visits.
• Field threshold estimates: Critical speed, functional threshold power, or a 30-minute time-trial average approximate LT2 without blood. They are practical and repeatable, though they estimate the boundary from performance rather than measuring blood lactate directly.
• Ventilatory thresholds: Gas-exchange analysis finds VT1 and VT2, the points where breathing patterns change as carbon dioxide rises. They track LT1 and LT2 closely and need no blood, which is why many metabolic carts report them instead.
• Heart-rate and talk-test proxies: The talk test (still conversational at LT1, clipped sentences near LT2) and threshold heart rate give a rough field guide. They drift with heat, fatigue, and caffeine, so treat them as estimates, not fixed numbers.

Types

Worked example

Here is a simplified incremental treadmill step test for a recreational runner. Blood is taken from the fingertip at the end of each 3-minute stage. Watch where the lactate curve bends: the small rise above baseline flags LT1, and the steep climb past a steady 4 mmol/L flags LT2.

The runner can hold roughly 15 km/h before lactate spirals, so that pace anchors their threshold sessions. Repeat the test after a training block and the whole curve shifts right, meaning the same lactate now appears at a faster speed. That rightward shift, not a change in VO2 max, is what usually improves race times.

Lactate threshold vs VO2 max

VO2 max is the size of your engine; lactate threshold is how much of that engine you can use for a sustained effort. Endurance performance depends on both, but two athletes with the same VO2 max are usually separated by whoever has the higher threshold.

By goal

• Beginners and general fitness: You do not need a lab. Spend most of your cardio at an easy, conversational pace below LT1 to build the aerobic base, and add one weekly session at a comfortably hard 'talk in short sentences' effort. That alone pushes your threshold higher over a few months.
• Endurance athletes: Test your thresholds, then train polarised: roughly 80% of volume easy below LT1 and 20% hard at or above LT2. Threshold or tempo intervals (for example 4 x 8 minutes near LT2) teach your body to clear lactate faster and hold a higher percentage of VO2 max.
• Health and longevity: Zone 2 work just below LT1 improves mitochondrial density, fat oxidation, and metabolic health with low fatigue cost. Two to four easy 45-to-60-minute sessions a week deliver most of the benefit and leave you fresh for strength training.

Common misconceptions

• "Lactic acid causes the burn during exercise and the soreness the next day." Neither. The burn during hard effort comes from hydrogen ions and other metabolites accumulating and dropping muscle pH, not from lactate. Delayed soreness (DOMS) 24 to 72 hours later is microscopic muscle damage and inflammation. Lactate is cleared within an hour and is not the culprit.
• "Lactate is a useless waste product your body dumps." Lactate is a fuel, not waste. Through the lactate shuttle it is carried to the heart, slow-twitch fibres, and liver, where it is oxidised for energy or rebuilt into glucose. Producing it also regenerates the NAD+ that keeps fast energy production running.
• "The lactate threshold is always 4 mmol/L." The fixed 4 mmol/L rule (OBLA) is a testing convenience, not a universal truth. An individual's true maximal lactate steady state can fall anywhere from about 2 to 8 mmol/L depending on the person, the sport, and the muscle mass involved. Blindly using 4 mmol/L can misjudge your real pace.
• "There is one single lactate threshold." There are two meaningful turning points. LT1, the aerobic threshold, is where lactate first rises above baseline near the top of Zone 2. LT2, the anaerobic threshold, is the ceiling of steady-state effort. Good training targets both, not a single line.

Related terms

Lactate Threshold FAQ

What is lactate threshold in simple terms?

Lactate threshold is the effort level where lactate starts building up in your blood faster than your body can clear it. Below it, you can keep going for a long time. Above it, the effort feels hard and you fatigue quickly, so it marks your sustainable pace ceiling.

What is the difference between LT1 and LT2?

LT1, the aerobic threshold, is where blood lactate first rises above its resting level, around 2 mmol/L, near the top of easy Zone 2. LT2, the anaerobic threshold, is the highest intensity you can hold while lactate still plateaus, and it best predicts endurance race pace.

Is lactate threshold the same as anaerobic threshold?

The term anaerobic threshold usually refers to LT2, the higher of the two lactate turning points. The name is a bit misleading because energy production is not suddenly all anaerobic there. Most coaches now prefer lactate threshold or maximal lactate steady state for accuracy.

Does lactic acid cause muscle soreness?

No. Lactate clears from your blood within about an hour of stopping, long before soreness peaks. The ache you feel one to three days later is delayed onset muscle soreness, caused by microscopic damage and inflammation in the muscle fibres, not by lactate or lactic acid.

What causes the burning feeling in hard exercise?

The burn comes from a build-up of hydrogen ions and other metabolites that lower the pH inside your working muscles during intense effort. Lactate appears at the same time, which is why it was blamed for decades, but the acidity itself, not the lactate, drives the sensation.

How do you measure lactate threshold?

The gold standard is an incremental step test where you exercise at rising intensities while small blood samples are taken from a fingertip or earlobe each stage. Plotting lactate against intensity reveals the breakpoints. Field methods like a 30-minute time trial or critical power estimate it without blood.

What lactate level is the threshold?

There is no universal number. Resting lactate sits around 1 mmol/L, LT1 appears near 2 mmol/L, and a fixed 4 mmol/L reference is often used for LT2. But true individual steady state ranges from about 2 to 8 mmol/L, so testing beats assuming a single value.

How do you improve your lactate threshold?

Build a large aerobic base with high-volume easy Zone 2 work, then add threshold or tempo intervals at or just below LT2, such as 4 by 8 minutes. This grows mitochondria, capillaries, and lactate-transport proteins, so you clear lactate faster and hold a higher pace before it accumulates.

Is lactate threshold or VO2 max more important?

For endurance events, lactate threshold usually predicts performance better. VO2 max sets your aerobic ceiling, but lactate threshold determines how much of that ceiling you can sustain. Two athletes with the same VO2 max are often separated by whoever holds the higher threshold pace.

What is Zone 2 and how does it relate to lactate threshold?

Zone 2 is the easy-to-moderate intensity band that tops out around LT1, where lactate is only slightly above baseline. Training there builds aerobic fitness and fat burning with low fatigue. It is the foundation that lets harder threshold sessions raise your LT2 over time.

References

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2. Cairns SP. Lactic acid and exercise performance: culprit or friend? Sports Medicine, 2006. PubMed 16573356
3. Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metabolism, 2018. PubMed 29617642
4. Nixon RJ, et al. The origin of the maximal lactate steady state (MLSS). BMC Sports Sci Med Rehabil, 2024. PubMed 38317240
5. Foucher CD, Tubben RE. Lactic Acidosis. StatPearls, NCBI Bookshelf
6. Why Does Lactic Acid Build Up in Muscles? And Why Does It Cause Soreness? Scientific American
7. Lactate threshold. Wikipedia