The Lactate Curve and What It Means for Your Training

If you've done a lactate test — stepped through progressively faster intervals while someone pricked your finger between each one — you've seen this curve. Blood lactate concentration on the vertical axis, pace or power on the horizontal. A line that stays low and flat for a while, begins to rise gently, then kicks upward steeply.

You know there are thresholds on the curve. This article goes deeper: what those thresholds represent physiologically, why there are two of them, and how the curve maps onto both racing and training. The previous article introduced the lactate threshold as one of the key determinants of running performance. Here we unpack the full curve.

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The curve itself

A lactate curve is generated by an incremental exercise test. You run at a series of increasing intensities, typically for 3–5 minutes per stage, with blood lactate measured between stages. The result is a plot of how blood lactate concentration changes as intensity rises.

At low intensities, the curve is flat. Your muscles are producing lactate — they always are, even at rest — but the rate of production is matched by the rate of consumption. Lactate is a normal product of glycolysis (the breakdown of glucose for energy) and is itself used as fuel by mitochondria, the structures within your cells that produce energy aerobically. At easy paces, production and consumption are in balance, and blood concentration stays low.

As intensity increases, the curve begins to rise. Glycolytic energy production accelerates, producing more lactate than the aerobic system can immediately process. Blood lactate climbs — first gently, then steeply.

The curve is individual. Two runners at the same pace may have very different lactate values. And the same runner's curve shifts with training — rightward and downward as fitness improves. The thresholds occur at faster paces. This is what "raising your threshold" actually means.

[Figure 1: The lactate curve — Blood lactate concentration rises nonlinearly with intensity, with two key inflection points.]

A note on thresholds

It is tempting to think of the two thresholds as sharp boundaries — switches that flip at a precise pace. They are not. The underlying physiology is a continuous spectrum. If your LT1 is measured at 5:00/km, running at 4:59 is not meaningfully different from running at 5:01. The increase in metabolic cost from 5:03 to 5:01 is roughly the same as from 5:01 to 4:59, even though one pair straddles the measured threshold and the other doesn't.

The thresholds are landmarks we can identify and measure on the curve, and they correspond to genuine physiological transitions — but those transitions are gradual, not instantaneous. This matters for how you interpret them: they are useful anchors for structuring training, not lines you must avoid crossing by a single second.

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The first threshold — LT1

The first landmark on the curve is the intensity at which blood lactate first rises measurably above its resting baseline. Below this point, aerobic energy production dominates and lactate balance is easily maintained. Above it, glycolysis is contributing more meaningfully to energy production, and the surplus begins to appear in the blood.

A subtlety worth noting: blood lactate is measured at a peripheral site — typically a fingertip or earlobe — not within the working muscle. By the time lactate appears in the blood, the local picture may have shifted earlier. Within the muscle, glycolytic contribution may already have increased, but the lactate produced is being consumed locally by mitochondria or shuttled to neighbouring muscle fibres and used as fuel there. Blood lactate is a downstream signal — real and informative, but not a direct window into what's happening at the cellular level.

LT1 is closely related to the concept of FatMax — the intensity at which fat oxidation is highest. The two are often used interchangeably, but they measure different things: LT1 marks the first detectable shift in lactate balance, while FatMax marks the peak rate of fat burning. In many runners they occur at similar intensities, but in well-trained athletes with high oxidative capacity, FatMax can sit meaningfully below LT1 — the aerobic system is efficient enough at processing lactate locally that the blood signal is delayed beyond the point where fat oxidation has already begun to decline.

What it means practically

LT1 marks the ceiling of genuinely easy running. Below it, metabolic stress is low, recovery cost is minimal, and you can sustain the effort for very long durations. This is the domain where easy runs and recovery runs should live.

The most common training error in amateur distance running is running easy sessions above LT1. The pace doesn't feel hard — it's comfortable, conversational even — but the metabolic cost is meaningfully higher than true easy running. This doesn't eliminate the training benefit of the session; if anything, the stimulus per session is greater. The problem is the fatigue cost. Easy runs exist to provide aerobic development at minimal recovery expense, preserving the athlete's capacity to execute hard sessions well and to absorb the full training week. Running them above LT1 erodes that bargain — more fatigue accumulated on days that were supposed to be cheap, leaving less capacity for the sessions that matter most.

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The second threshold — LT2

The second landmark is where the curve inflects sharply upward — the intensity at which lactate accumulation accelerates steeply. This represents the highest intensity at which lactate production and clearance can remain roughly in balance over time. Above it, lactate and associated metabolic byproducts accumulate progressively, and the effort becomes unsustainable.

This is the threshold referenced in the Joyner performance model — the one that governs fractional utilisation of VO₂max. It is what most coaches mean when they say "lactate threshold" without further qualification.

What it means practically

LT2 is the boundary between sustainable and unsustainable racing intensity. A well-trained runner can sustain effort at or just below LT2 for roughly 30–60 minutes depending on fitness and event experience. This is why LT2 is so closely associated with performances from 10k to half-marathon distance — these races are run near or at this boundary.

The distance between LT1 and LT2 — both in terms of pace and in terms of the metabolic space between them — is itself informative. As an athlete's aerobic system matures, both thresholds shift rightward, but the gap between them tends to widen. A well-trained endurance athlete typically has a broad range of intensities that are above easy but still metabolically manageable — a wide zone of sustainable moderate work. A less developed runner may have the two thresholds compressed together, sometimes because LT1 is barely distinguishable from resting levels in someone whose aerobic base is still undeveloped. As training builds that base, LT1 emerges as a distinct landmark and the usable space between the two thresholds opens up.

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The three intensity domains

The two thresholds divide the intensity spectrum into three primary domains. Each has distinct physiological characteristics, distinct training applications, and a distinct role in the overall training structure.

A note on terminology: the exercise physiology literature refers to these domains as moderate (below LT1), heavy (between LT1 and LT2), and severe (above LT2). We use easy, moderate, and severe — names that better describe how these intensities feel and function in a training context. The underlying physiology is the same regardless of labelling.

[Figure 2: The three intensity domains — LT1 and LT2 divide the intensity spectrum into easy, moderate, and severe domains.]

Below LT1 — Easy

Low metabolic stress. Highly sustainable. Minimal recovery cost. This is the domain of easy runs, the aerobic portions of long runs, and recovery sessions. The primary adaptations driven by work in this zone are the aerobic foundations: mitochondrial development, capillarisation, cardiac adaptations — built at a fatigue cost low enough to allow high training volumes.

For higher-volume athletes, the majority of weekly training sits here — and for good reason. Easy running provides meaningful aerobic stimulus at a fatigue cost that preserves the athlete's capacity to execute hard sessions well and to train again the next day. It is the foundation that makes the rest of the training week possible.

Between LT1 and LT2 — Moderate

Lactate is elevated but manageable. Effort feels controlled and purposeful — harder than easy, clearly sustainable for extended periods but not indefinitely. This zone includes tempo efforts, threshold intervals, and marathon-pace work for most amateur runners.

The Norwegian double-threshold model operates in this domain — with a morning session targeting the lower-to-middle portion of the zone and the afternoon session targeting the upper portion, both deliberately staying below LT2. The rationale is to accumulate a high volume of quality work without crossing into intensities that impose heavy fatigue and long recovery.

The primary adaptations: the enzymatic and structural changes that shift both thresholds rightward — improved mitochondrial function, enhanced capillary density, better lactate processing capacity. This is the zone where threshold fitness is built.

Above LT2 — Severe

Lactate accumulates progressively. Duration is limited by the rate of accumulation — the further above LT2, the shorter the sustainable effort. This domain includes VO₂max intervals, race-pace work for distances from 3k to 10k, and speed development.

The primary adaptations: VO₂max development, anaerobic capacity, and neuromuscular power. Work here is potent but fatiguing, which is why it occupies a small fraction of total training volume — typically one or two sessions per week, even for serious athletes.

Various coaching systems further subdivide these domains into five, six, or seven zones. Those finer divisions are useful for detailed prescription but rest on this three-domain foundation — and understanding the foundation is more useful than memorising zone numbers.

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Where race distances live on the curve

Different race distances demand effort at different points on the curve. Understanding where your target event sits clarifies both what limits your performance and what your training should emphasise.

[Figure 3: Where race distances sit on the curve — Race intensity relative to the thresholds varies by distance and by athlete.]

Marathon. For most amateur runners, marathon pace sits between LT1 and LT2 — in the moderate domain. Where exactly depends on the athlete: a 3:30 marathoner might race closer to LT1, while a 2:20 marathoner races much nearer LT2. Marathon performance is therefore highly sensitive to both thresholds and to durability — the race is run in the zone where threshold fitness governs what's sustainable, and it lasts long enough for physiological degradation to become decisive.

Half marathon. Sits near or just below LT2 for most trained runners. The half is often considered the purest test of threshold fitness — it demands sustained effort very close to the second threshold for an extended period. Runners who have invested in raising LT2 tend to see disproportionate returns at this distance.

10k. At or slightly above LT2 for well-trained runners. Short enough that running above the sustainable ceiling is manageable, long enough that metabolic accumulation cannot be ignored. A blend of threshold fitness and VO₂max.

5k and 3k. Clearly above LT2, increasingly into the severe domain. VO₂max becomes a more prominent determinant. The ability to sustain a high fraction of your aerobic ceiling — and to tolerate the metabolic consequences of exceeding it — is what separates performances.

The practical implication: your target distance should inform which part of the curve your training emphasises. A marathoner needs to shift both thresholds rightward and develop durability at intensities between them. A 5k runner needs a high VO₂max ceiling and the capacity to race in the severe domain. Both benefit from the aerobic base built below LT1 — but the specificity of their quality sessions points to different parts of the curve.

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The lactate curve is not just a diagnostic tool — it is a map of the intensity spectrum that connects physiology to training to racing. Understanding where the landmarks sit, what they represent, and how your target event relates to them is foundational to designing training that addresses the right systems.

The physiology article covers where the threshold fits within the broader performance model. How Your Body Fuels Running goes deeper into the energy systems — glycolysis, oxidative phosphorylation (the aerobic process within mitochondria that produces the bulk of your energy during endurance exercise), and the substrate dynamics that underlie everything discussed here.

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References and Further Reading

The Lactate Curve and Threshold Concepts

• Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Medicine. 2009;39(6):469–490. — A critical review of the various threshold definitions and their validity as performance predictors.

• Beneke R, Leithäuser RM, Ochentel O. Blood lactate diagnostics in exercise testing and training. International Journal of Sports Physiology and Performance. 2011;6(1):8–24. — A comprehensive guide to lactate testing methodology and interpretation.

• Jamnick NA, Pettitt RW, Granata C, Pyne DB, Bishop DJ. An examination and critique of current methods to determine exercise intensity domains in trained subjects: a systematic review. Sports Medicine. 2020;50(8):1443–1480. — A systematic review of how intensity domains are defined and measured, including the moderate/heavy/severe framework.

Intensity Distribution and Training Zones

• Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance. 2010;5(3):276–291. — The foundational review on how elite endurance athletes distribute training across intensity zones.

• Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high-intensity, or high-volume training. Frontiers in Physiology. 2014;5:33. — A controlled comparison of intensity distribution models.

The Norwegian Model

• Casado A, Hanley B, Jiménez-Reyes P, Renfree A. 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. — A detailed description of the lactate-guided threshold model and its physiological rationale.

• Haugen T, Sandbakk Ø, Seiler S, Tønnessen E. The training characteristics of world-class distance runners: an integration of scientific literature and results-proven practice. Sports Medicine – Open. 2022;8:46. — How elite distance runners actually train, including the Norwegian approach in context.

Fat Oxidation and LT1

• Achten J, Jeukendrup AE. Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine. 2003;24(8):603–608. — The original FatMax research establishing the relationship between exercise intensity and peak fat oxidation.

• Bircher S, Knechtle B. Relationship between fat oxidation and lactate threshold in athletes and obese women and men. Journal of Sports Science & Medicine. 2004;3(3):174–181. — Evidence on how the FatMax/LT1 relationship differs between trained and untrained populations.

Race Performance and Physiology

• Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. Journal of Physiology. 2008;586(1):35–44. — The foundational review on the physiological determinants of endurance performance.

Exercise Intensity Domains

• Burnley M, Jones AM. Oxygen uptake kinetics as a determinant of sports performance. European Journal of Sport Science. 2007;7(2):63–79. — Foundational work on the moderate/heavy/severe domain framework and the physiological transitions that define each.