Ventilatory and Lactate Thresholds: What LT1, LT2, VT1, and VT2 Actually Mean

By Marc Ferrer · Founder, Engram Kinetics · Published August 2026 · Updated October 2026

Few topics in exercise physiology are buried under as much terminology as the “thresholds.” A single graph gets labeled, depending on the textbook, with aerobic threshold, anaerobic threshold, lactate threshold, LT1, LT2, ventilatory threshold, VT1, VT2, gas exchange threshold, respiratory compensation point, OBLA, MLSS, and (in the more recent literature) critical power and critical speed. Readers reasonably conclude there must be a dozen different things happening. There are not. Underneath the vocabulary there are essentially two transitions, and most of those terms are just different names, measured by different methods, for one or the other of them.

This article untangles the naming. What the two transitions actually are, which terms belong to which, how the lactate and ventilatory markers relate, why the lactate version and the ventilatory version of a threshold are close cousins but not the same number, and (the part the older textbooks miss) how the field has moved on from fixed lactate values toward critical power as the modern reference for the upper boundary. Get the map straight once and the whole topic stops being a memorization slog.

The Two Transitions

Picture exercise intensity rising from an easy walk to an all-out effort. Two distinct things happen to metabolism along the way, and they carve the intensity range into three zones.

The first transition is the intensity at which blood lactate first climbs meaningfully above its resting baseline. Below it, you are in the moderate-intensity domain: lactate stays near baseline, effort feels easy, and you could sustain it for a very long time. This first transition marks the exit from that comfortable domain. In lactate terms it is the first lactate threshold (LT1), often loosely called the aerobic threshold.

The second transition is the ceiling of sustainability: the highest intensity at which the metabolic disturbance can still stabilize rather than climbing to exhaustion. Below it, lactate and oxygen uptake are elevated but can plateau at a new steady level; above it, they drift relentlessly upward until you have to stop. In lactate terms this transition is the second lactate threshold (LT2), historically anchored to the maximal lactate steady state (MLSS): the highest workload you can hold with lactate stable rather than climbing. As we’ll see, the most current way to define this ceiling is not a lactate value at all but critical power.

So: two transitions, three domains. Moderate below the first, heavy between the two, severe above the second. (A fourth, “extreme” domain is sometimes added at the very top, where exhaustion arrives before oxygen uptake can reach its maximum.) Almost every threshold term you will meet is a label for the first transition or the second, or a specific method of finding one of them.

The Lactate Family

Measured from blood lactate, the two transitions get the cleanest names.

LT1 is the first sustained rise above baseline. It is genuinely individualized: it is defined by the shape of a person’s lactate curve, not by hitting a fixed number. You will sometimes see it approximated at a blood lactate of around 2 mmol/L, but that is a rough convention, not the definition. A trained endurance athlete and a deconditioned beginner can both have an LT1, at very different intensities, and neither is guaranteed to land at exactly 2 mmol/L.

LT2 is the second, steeper rise, an incremental-test marker for the point beyond which lactate can no longer stabilize. The criterion version of that ceiling is the maximal lactate steady state (MLSS): measured from constant-load exercise as the highest workload at which blood lactate holds steady rather than climbing. LT2 estimates that boundary, but an incremental-test LT2 and a constant-load MLSS do not perfectly coincide in every person, which is worth remembering whenever a single test is treated as the definitive second threshold. Here too, a fixed-value shortcut circulates: OBLA, the onset of blood lactate accumulation, places the second threshold at a blood lactate of 4 mmol/L. And here too, the fixed number is a convenient surrogate, not the truth. An individual’s true MLSS can sit above or below 4 mmol/L, and the incremental markers can land above or below it too; assuming everyone’s second threshold lives at exactly 4 mmol/L will misclassify a meaningful fraction of people.

This is where the field has genuinely moved, and where older textbooks lag. The fixed values (2 mmol/L for the first threshold, 4 mmol/L for the second) are now regarded as dated as universal criteria, not merely imperfect. Modern critiques (Faude and colleagues in 2009, and more forcefully Jamnick and colleagues in 2020) showed that the lactate response is too individual and too dependent on the test protocol for a fixed number to be valid across people; OBLA at 4 mmol/L in particular tends to overestimate the true MLSS. Where individualized lactate methods are used, approaches like the Dmax method or the individual anaerobic threshold agree with MLSS better than a fixed cut-off does. The tidy round numbers survive only as rough field shortcuts, and treating them as the real threshold is exactly the kind of dated practice this article is meant to help you avoid.

The Ventilatory Family

You can find the same two transitions without drawing blood, by watching what breathing does. As metabolic acidosis develops, ventilation responds in two stages, and those two stages are the ventilatory thresholds.

VT1, the first ventilatory threshold, is also called the gas exchange threshold (GET); the two are used interchangeably here, though strictly they emphasize slightly different detection methods for the same transition. It is the intensity at which carbon dioxide output (VCO2) begins to rise faster than oxygen uptake (VO2). On a gas-exchange test it shows up as the point where the ventilatory equivalent for oxygen (the ratio of ventilation to VO2) starts to climb while the ventilatory equivalent for carbon dioxide stays flat or continues to fall; equivalently, the “V-slope” breakpoint where the VCO2-versus-VO2 line steepens. The classical explanation is that bicarbonate buffering of the rising acid load releases additional CO2, which contributes to the first bump in ventilation, though ventilatory control has several inputs, not this one alone. VT1 corresponds, in intensity, to roughly where LT1 sits.

VT2, the second ventilatory threshold, is also called the respiratory compensation point (RCP). It is the intensity at which ventilation increases out of proportion even to the rising CO2, where the ventilatory equivalent for carbon dioxide itself begins to climb, and end-tidal CO2 begins to fall, because breathing is now being driven hard to compensate for a developing metabolic acidosis. VT2/RCP corresponds, in intensity, to roughly where LT2/MLSS sits.

So the ventilatory family mirrors the lactate family: VT1/GET tracks the first transition, VT2/RCP tracks the second. Two methods, one underlying pair of events.

Why the Lactate and Ventilatory Versions Are Not the Same Number

Here is the part that gets glossed over, and the part most worth understanding. It is tempting to write LT1 = VT1 and LT2 = VT2 and be done. That is close enough to be useful and wrong enough to matter.

LT1 and VT1 are measuring related but distinct physiological events (one is a change in blood lactate concentration, the other a change in the ventilatory response to CO2), and they are linked by a chain of physiology (lactate buffering producing CO2 driving ventilation) that is a well-supported association rather than a guaranteed identity. In a given individual on a given day, the ventilatory breakpoint and the lactate breakpoint often land at similar intensities, but they can and do diverge. The same holds for LT2 and VT2/RCP. Treating the two methods as interchangeable (assuming the number you got from gas exchange is exactly the number you would have gotten from blood lactate) is a common oversimplification that the more careful literature specifically warns against.

Practically, this means the method matters. A threshold identified by lactate sampling and a threshold identified by ventilatory gas exchange are estimates of the same underlying transition by two different routes, and they carry different error and can disagree. When a study, a coach, or an exam item hands you “the threshold,” the first question is which one, found how.

The Second Boundary Has a Newer Name: Critical Power

For decades, the maximal lactate steady state was treated as the gold-standard definition of the second, upper transition. In the last several years that has shifted, and it is the most important update to this whole topic. A body of work led by Andrew Jones, David Poole, and colleagues has argued that the true boundary of sustainable exercise (the real dividing line between the heavy and severe domains) is best captured by critical power (CP), or its running/swimming equivalent critical speed (CS), rather than by MLSS.

Critical power comes from a different kind of test. Instead of an incremental ramp or a series of constant-load lactate trials, you have the athlete perform several all-out or fixed-intensity efforts to exhaustion at different intensities. The relationship between power output and time-to-exhaustion is a hyperbola, and critical power is the asymptote of that hyperbola: the highest power output that can, in principle, be sustained without the body’s key metabolic signals (phosphocreatine, pH, blood lactate, oxygen uptake) drifting to their limits. Below CP, those signals stabilize within a couple of minutes; above CP, none of them stabilizes, and exhaustion becomes a matter of time. The finite amount of work you can do above CP before that happens is a companion quantity called W′ (W-prime).

Here is the part that matters for the map. Critical power and MLSS are close but not the same: the current evidence places CP slightly above MLSS, by a modest margin that varies with fitness and age, and the two are not interchangeable in an individual. In the modern view, CP is the more complete definition of the maximal metabolic steady state, and MLSS sits just underneath it as a closely related, lactate-based estimate. The ventilatory second threshold (RCP/VT2) is another neighbor: at the group level it tends to fall near CP, but at the individual level it is an imperfect surrogate and can diverge, especially in clinical or low-oxygen conditions.

So the honest, current picture of the second transition is a small cluster of related-but-distinct markers (LT2, MLSS, RCP/VT2, and critical power), with critical power now treated as the leading reference and the others as estimates that usually land nearby but not exactly on it. This is a genuinely evolving area rather than settled dogma: the CP-as-gold-standard argument dates from around 2019 and has drawn published debate. But a candidate or coach working from a textbook that stops at “the second threshold is MLSS at 4 mmol/L” is working from a map two revisions out of date.

The “Anaerobic Threshold” Problem

One term deserves special caution: anaerobic threshold. It is historically important (it comes from the foundational gas-exchange work of Wasserman and colleagues), and it is genuinely confusing, for two reasons.

First, it has been used to mean different things: sometimes the single-threshold idea, sometimes specifically the second transition (LT2/VT2). If someone says “anaerobic threshold” without specifying, you often cannot tell which transition they mean.

Second, and more important, the name embeds a claim that is not accurate. The muscle does not switch to “anaerobic” metabolism at this intensity. Lactate is produced continuously, at rest and during fully aerobic exercise; the threshold is not the point where anaerobic metabolism switches on. Blood lactate accumulates when its rate of appearance outpaces its rate of clearance: a balance between production and removal, not a failure of oxygen supply. For this reason much of the modern literature prefers the descriptive, mechanism-neutral names (gas exchange threshold, respiratory compensation point, lactate threshold) over “anaerobic threshold,” which names a mechanism that is not really what is happening.

This is not a fringe objection. In 2021 Poole and colleagues published paired papers, one titled, pointedly, “The ‘Anaerobic Threshold’ Concept Is Not Valid in Physiology and Medicine,” arguing that the term should be retired outside of history, and that the field should speak instead of the lactate threshold, the gas exchange threshold, and critical power, expressed where possible as an oxygen uptake rather than a work rate.

You still need to recognize the term, because it is everywhere and will be on study materials for years. You just should not take its literal meaning at face value.

Why This Matters for Prescription, and the Exam

The two transitions are not academic trivia; they define the intensity domains that exercise prescription is built on. Work below the first threshold is sustainable for hours and anchors long-duration, base-building training. Work between the thresholds is harder but still steady-state-able for a meaningful time. Work above the second threshold is inherently limited: VO2 drifts upward toward maximum if the effort is sustained, and it cannot be held for long. Training zones, whether they are anchored to heart rate, power, pace, or a percentage of VO2max, are ultimately attempts to place a session in one of these domains.

And this is the second place the modern literature has moved, alongside the retirement of fixed lactate values. Current work (Jamnick and colleagues in 2020, Iannetta and colleagues the same year) argues that intensity should be prescribed by anchoring to an individual’s actual thresholds (the first and second transitions, measured) rather than to a fixed percentage of maximum heart rate or VO2max. The reason is direct: the same percentage of maximum lands different people in different domains, because their thresholds sit at different fractions of their maximum. A prescription of “70% of VO2max” is moderate for one person and heavy for another. Anchoring to the transitions themselves is what keeps a prescribed intensity in the domain it was meant to target. (Worth noting separately: a 2025 joint ACSM and Exercise & Sports Science Australia statement standardized a simple public-facing intensity scale for communication, a distinct, coarser tool that should not be confused with the three physiological domains described here.)

Which is exactly why the terminology confusion is not harmless. If a prescription is anchored to “the anaerobic threshold” and one source means LT1 while another means LT2, the resulting training intensity can be wrong by a whole domain. If a field test pins the second threshold at a fixed 4 mmol/L for a client whose true MLSS sits elsewhere, the zones built on it are off. And if a candidate treats a ventilatory-derived number and a lactate-derived number as identical, they will misread items that hinge on the distinction.

On the exam, threshold items rarely reward reciting a definition. They reward knowing which threshold a scenario refers to, how it was measured, and whether a fixed-value proxy is being used where an individualized threshold was needed. That is a decision about what the number in front of you actually represents: the same interpretive skill that separates knowing a value from knowing what to do with it.

FAQ

Is the lactate threshold the same as the ventilatory threshold? They estimate the same underlying transitions by different methods, and they usually land at similar intensities, but they are not guaranteed to be identical. LT1 and VT1/GET both mark the first transition; LT2 and VT2/RCP both mark the second. In an individual they can diverge, so the measurement method matters and the two should not be treated as the same number.

What’s the difference between LT1 and LT2? LT1 is the first meaningful rise in blood lactate above baseline: the exit from the easy, moderate-intensity domain. LT2 is the highest intensity at which lactate still stabilizes rather than climbing relentlessly, closely associated with the maximal lactate steady state. LT1 is the lower intensity; LT2 is the upper ceiling of sustainable work.

Are the 2 mmol/L and 4 mmol/L values real thresholds? No, and this is one of the more out-of-date ideas in the older textbooks. They are fixed-value proxies, not definitions, and modern reviews regard them as dated for individual use. The real thresholds are individualized features of a person’s lactate curve. Around 2 mmol/L is sometimes used to approximate LT1 and 4 mmol/L (OBLA) to approximate LT2, but people vary, a person’s actual threshold can sit meaningfully above or below these round numbers, and OBLA in particular tends to overestimate the true maximal lactate steady state. Individualized methods (such as the Dmax method or the individual anaerobic threshold) agree with the real boundary better than a fixed cut-off does.

What is critical power, and how does it fit in? Critical power (or critical speed, for running and swimming) is the current leading definition of the second, upper transition: the true boundary of sustainable exercise. It comes from the relationship between intensity and time-to-exhaustion, and it sits slightly above the maximal lactate steady state. In the modern picture, the second threshold is a small cluster (LT2, MLSS, RCP/VT2, and critical power), with critical power treated as the reference and the others as nearby estimates. A textbook that stops at “the second threshold is 4 mmol/L” is two revisions behind.

Why is “anaerobic threshold” discouraged? Because the muscle is not actually anaerobic at that intensity: lactate is produced aerobically, and it accumulates because clearance is outpaced, not because oxygen ran out. The term also gets used ambiguously for different transitions. Mechanism-neutral names (gas exchange threshold, respiratory compensation point, lactate threshold) are clearer, though you still need to recognize “anaerobic threshold” because it remains in wide use.

Key Takeaways

Under a mountain of terminology there are two transitions. The first (LT1 in lactate terms, VT1 or gas exchange threshold in ventilatory terms, loosely “aerobic threshold”) marks the exit from the easy moderate-intensity domain, where lactate first rises above baseline. The second (LT2 in lactate terms, VT2 or respiratory compensation point in ventilatory terms, historically anchored to the maximal lactate steady state and, in the current literature, best defined by critical power) marks the ceiling of sustainable work, above which the metabolic disturbance drifts relentlessly. The lactate and ventilatory markers estimate the same two events by different methods and usually agree closely, but they are not guaranteed identical, so the measurement method matters. Two things have genuinely dated in the older textbooks: the fixed lactate values (2 and 4 mmol/L) are now treated as rough shortcuts rather than real thresholds, and the second boundary is now referenced to critical power, with MLSS and RCP/VT2 as nearby estimates. Modern prescription anchors intensity to a person’s actual thresholds rather than to a fixed percentage of maximum. And “anaerobic threshold,” though everywhere, names a mechanism that is not really what happens and is being actively retired. Get which term means which transition straight, and the whole tangle resolves into two lines on a graph.

Disclosure: Marc Ferrer is the founder of Engram Kinetics, an independent exam-prep platform. Engram Kinetics is not affiliated with, endorsed by, or sponsored by ACSM, NSCA or CSEP.