Lactate Threshold: What It Is and How to Test It

Ask three coaches to explain lactate threshold and you'll likely get three overlapping but not quite identical answers. Ask how it relates to anaerobic threshold, or to FTP, and the overlap gets messier still, partly because triathlon borrows vocabulary from exercise physiology, cycling performance labs, and running coaching traditions that don't all use the same words for the same thing. That's not you misunderstanding something simple. It's a real, acknowledged inconsistency in how the sports science and coaching worlds talk about this stuff.

This piece untangles it: what's happening physiologically, how it's measured in a lab, what triathletes use instead in the real world, and, honestly, where the terms genuinely overlap and where they don't.

Why the vocabulary got this messy

Part of the confusion has a real history behind it. Exercise physiologists were mapping lactate response curves and ventilatory changes during graded exercise back in the 1960s and 70s, working out where breathing rate and blood lactate both shift as effort climbs. Bertil Sjödin's OBLA research, using a fixed blood lactate cutoff, followed in the early 1980s. Cycling's FTP framework came along decades later, popularized in the early 2000s by Andrew Coggan and Hunter Allen as a power-based training system built for practicality on a bike computer, not as a literal restatement of the lab concept. Running coaches developed their own pace-based and heart-rate-based threshold testing traditions on a similar track, independently of the cycling world.

Three overlapping vocabularies, built by different communities for different purposes at different times, is exactly the kind of thing that produces inconsistent terminology. None of the groups involved were wrong. They were solving adjacent problems and reusing nearby language, and triathlon, a sport that borrows from all three traditions at once, inherited the mismatch.

What's happening in your blood

During exercise, your muscles produce lactate as a normal byproduct of energy metabolism, at every intensity, not just hard ones. At easy efforts, your body clears lactate about as fast as it's produced, so blood lactate stays low and stable. As intensity climbs, production eventually starts outrunning clearance, and blood lactate begins to rise.

That rise doesn't happen in one clean jump. Exercise physiologists generally describe two separate inflection points on the way up:

• A first, lower rise above resting levels, often called the aerobic threshold or LT1, where lactate starts climbing modestly above baseline.
• A second, higher point where lactate stops holding steady altogether and starts climbing continuously, often called the anaerobic threshold, LT2, or maximal lactate steady state (MLSS): the highest intensity your body can sustain where production and clearance still roughly balance.

Both of these are sometimes called "lactate threshold" depending on who's talking and in what context, which is a real source of the confusion. In everyday triathlon coaching conversation, when someone says "lactate threshold" or "anaerobic threshold," they're almost always pointing at the second, higher one: the ceiling above which sustained effort gets short-lived fast.

How it's measured in a lab

The gold-standard method is a graded exercise test: you run or ride at increasing intensity in stages, usually a few minutes each, while a technician draws a small blood sample from your fingertip or earlobe at the end of every stage to measure lactate concentration. Plot those values against intensity and you get a curve that stays flat, then bends upward.

Where exactly that bend counts as "threshold" is where the science gets genuinely contested. One long-standing convention, the onset of blood lactate accumulation (OBLA), uses a fixed 4 mmol/L cutoff, introduced by Bertil Sjödin's research in the early 1980s. It's simple and widely used, but researchers have since shown that a fixed 4 mmol/L point doesn't land in the same physiological spot for every athlete, or even for the same athlete under a different test protocol, which is exactly why alternative individualized methods, like the modified Dmax method or a fixed offset above baseline, exist and don't always agree with OBLA or with each other on the same data. This isn't a case of "science hasn't figured it out yet." It's closer to "several defensible methods exist, and honest researchers disagree on which one best tracks the underlying MLSS."

A typical protocol runs stages of three to five minutes each, long enough for lactate to reach a new steady state before the next step up, with power or pace rising by a fixed amount each stage until you can't hold the pace anymore or your lactate is climbing steeply. Some labs pair the blood draws with a mask measuring oxygen and carbon dioxide exchange, which lets them also mark the ventilatory threshold, the point where breathing rate jumps disproportionately to effort. That point tends to track closely with the lactate-based threshold, which is part of why some coaches use breathing pattern, the well-known "talk test," meaning you can speak in short sentences but not hold a full conversation, as a rough, zero-equipment proxy for the same intensity zone.

Two things worth being upfront about: this test is rarely painful, but it is uncomfortable at the harder stages, and it requires lab access most age-group triathletes don't have on a regular basis. That's exactly why almost nobody trains directly off a lab lactate curve.

Where anaerobic threshold and FTP fit

This is the part worth being precise about, because getting it wrong either overstates how exact these numbers are or understates how useful they still are.

Anaerobic threshold is the physiological concept: the intensity where lactate accumulation outpaces clearance, as Triathlify's anaerobic threshold entry covers in more depth. It's frequently used as a near-synonym for lactate threshold in its higher-intensity sense (LT2 / MLSS), and in casual coaching talk, treating them as the same thing rarely causes real confusion. Where it matters is remembering that neither term describes a single universally agreed lab value; both describe the same underlying crossover, measured by methods that don't perfectly agree with each other.

FTP, as Triathlify's FTP entry explains, is a cycling-specific field estimate: the highest power you could hold for about an hour, usually estimated from a 20-minute maximal effort multiplied by roughly 0.95, or from a ramp test. It's built to sit near the same intensity as anaerobic threshold and MLSS, and for most athletes it's close enough to be useful for setting training zones. But "close" isn't "identical." A 2019 study in the International Journal of Sports Physiology and Performance found FTP correlated strongly with lab-measured MLSS in trained cyclists, with a small average bias but real individual scatter. A 2023 study in the Journal of Sports Sciences pushed back harder, concluding FTP isn't a reliable marker of the threshold separating sustainable from unsustainable effort for every athlete tested. Both are legitimate, peer-reviewed findings pointing at the same open question: FTP is a good practical estimate, and it is not a guaranteed stand-in for a lab measurement.

Lactate threshold vs anaerobic threshold vs FTP, side by side

The field tests triathletes use

Cycling: the 20-minute FTP test. Warm up thoroughly, then hold the hardest sustainable effort you can for 20 minutes, ideally on a flat road or a trainer where power holds steady. Multiply the average power by roughly 0.95 to estimate FTP. A ramp test, riding at steadily increasing power until you can't continue, is the other common option, estimating FTP from where the power curve breaks down rather than from a sustained effort.

Running: the 30-minute time trial. Warm up for 10 to 15 minutes, then run at a hard, steady, race-like effort, not an all-out sprint, for 30 minutes. Start a lap split 10 minutes in and keep running hard for the remaining 20. Your average heart rate over that final 20 minutes is your threshold heart rate; your average pace across the full 30 minutes is your threshold pace. The reason the first 10 minutes get thrown out is that heart rate takes several minutes to climb to a true steady state at a new intensity, so including it would drag the average down and understate your actual threshold.

Swimming: critical swim speed. The idea carries over, but the standard test looks different: typically a time trial over a fixed distance (400m is common) rather than a fixed duration, since pacing a 20 or 30-minute maximal swim accurately is a harder ask than doing the same on a bike or a run.

All three share the same design logic: estimate a sustainable-but-hard effort from a test short enough to pace correctly, then use that number, not a lab value you probably don't have, to build training zones.

Using this without overthinking it

None of these numbers, lab-measured or field-tested, are meant to be treated as fixed facts about your body. Fitness changes, so FTP and threshold pace typically get retested every 6 to 8 weeks or after a focused training block, exactly like Triathlify's FTP entry recommends. Training off a stale number quietly misdefines every zone built from it, turning what's supposed to be a threshold interval into an easier tempo session in disguise, or the reverse.

The other trap is chasing false precision. A field-test number that's within a percent or two of a lab value is doing its job. Whether your true anaerobic threshold sits at exactly 4.0 mmol/L or 3.6 mmol/L on a curve you'll never see changes nothing about how you should train next week. What matters is having a consistent, repeatable number, tracked over time against your own trend, that tells you roughly where the line is between sustainable and unsustainable effort in each discipline.

If you're also tracking recovery alongside performance, it's worth being clear that resting heart rate and threshold heart rate are unrelated numbers measuring different things: one reflects your cardiovascular system at rest, the other your ceiling under hard, sustained effort. Both are useful. Neither substitutes for the other.

Sources: Sjödin & Jacobs (1981) on OBLA methodology; INSCYD's overview of the 4 mmol/L threshold's individual variability; Borszcz et al. (2019), International Journal of Sports Physiology and Performance, on FTP vs. MLSS agreement in trained cyclists; a 2023 Journal of Sports Sciences study questioning FTP as a maximal metabolic steady state marker; Joe Friel's 30-minute field test protocol for running and cycling threshold heart rate.

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