Lactate Thresholds Explained: LT1, LT2 and Why Two Numbers Matter

Short answer: as you speed up, there are two points where your body's response to exercise changes. The first, LT1 (often called the aerobic threshold), is where blood lactate first rises above its resting level. It is the top of your easy training. The second, LT2 (close to what coaches call "threshold"), is roughly the highest effort at which lactate still settles at a steady level (1). It is the anchor for your hard sessions. Know both: the upper one tells you how hard your intervals should be, the lower one caps your easy days.

Many amateurs know only one number, usually FTP (the highest power you can hold for about an hour) or a threshold pace. That tells you your top steady effort, not where your easy days should stop, which is where most training hours are spent.

Which numbers should you actually know?

For most amateurs, the recommendation is simple: get your upper threshold from a field test, and find your lower threshold by feel, using the talk test.

• Upper threshold (LT2): a 20-minute bike test for FTP, or a 30-minute solo run for your lactate threshold heart rate (LTHR) and threshold pace (2). The protocol is below.
• Lower threshold (LT1): the point where talking starts to take effort: you can still speak in full sentences, but not comfortably. A review of subjective methods found that this "just about able to talk" point lines up with the first ventilatory threshold (where breathing first picks up) (3), which sits close to LT1 (4). On average it also matches about 10–11 on the 6–20 effort scale, which reads "light", though individuals vary (3). Easy days sit just below it: full sentences, comfortably.

What matters most depends on where you are:

• If you're newer to endurance training, LT1 is the more useful number. Many coaches consider keeping easy sessions truly easy one of the most useful changes you can make, and you can do it without any test at all.
• If you're experienced, you probably know LT2 already. Check that your "easy" runs and rides really sit below LT1, and decide on purpose how much time you spend between the two.

Not sure where to start? The talk test takes one session. A lab test is more precise but not necessary for most amateurs, and both numbers move with fitness, so update them every few months.

What are LT1 and LT2, in plain words?

Your muscles produce lactate all the time, and your body clears it. At easy efforts, clearance keeps up and blood lactate stays near resting levels. That is the zone below LT1: conversational, sustainable for hours.

Push a little harder and lactate rises, then settles at a higher but stable level. Breathing deepens and the effort feels "steady". You can hold it for a long time, but it costs more recovery than easy work.

Push harder still and you reach LT2: the highest effort at which lactate can still settle. A little above it, nothing settles: lactate, breathing and heart rate keep drifting upward, and the time you can keep going is limited (5).

On a lab report or in an app you may see other names:

• VT1 (first ventilatory threshold) is close to LT1 (4).
• MLSS (maximal lactate steady state) is close to LT2.
• VT2 (second ventilatory threshold) usually sits above both, so don't use it as your FTP (6).
• Critical power (critical speed for runners) usually sits above LT2 and MLSS: in one meta-analysis, MLSS was about 11% below critical power on average (6). If your app shows it, treat it as an upper limit for steady efforts, not as your FTP.

FTP and LTHR are field estimates in the same region as LT2; one of them is enough as your upper anchor.

Why two numbers instead of one?

Most zone systems use a single anchor, such as FTP, LTHR or maximum heart rate. That works well for hard sessions near the anchor but less well for finding where easy ends: in one review of intensity methods, fixed percentages of a maximum did not reliably put different people in the same intensity band (7). Many coaches find the same problem when easy days are set as a fixed share of FTP.

The two numbers also give the band between them a job. As a coaching rule of thumb, faster amateurs race a marathon or a half-Ironman bike leg in this band. Slower finishers are often close to LT1, and full-Ironman pacing usually sits around or just below it. So practise race pace in long sessions, and keep the rest of your easy days below LT1. Many coaches see trouble when easy days drift into the band by accident.

How to use LT1 and LT2 in a real training week

Here is an example week of about 7½–8 hours for a self-coached triathlete, with effort-based alternatives for anyone without numbers:

• Monday: easy 30-minute swim, or rest.
• Tuesday: 75-minute ride: 20 minutes easy warm-up, 3 × 10 minutes at or just under FTP with 5 minutes easy between, 15 minutes easy to finish. Effort: hard but controlled, short phrases only. If you are down to single words, you have gone too hard, so ease off.
• Wednesday: easy 50-minute run below LT1.
• Thursday: 45-minute swim with 8 × 100 metres at your critical swim speed (CSS, your threshold pace in the pool), 15–20 seconds' rest between. Without a CSS, swim them hard but controlled: you could do one more at the same pace. Add an easy 45-minute ride below LT1.
• Friday: rest or mobility.
• Saturday: long ride of 2.5–3 hours, mostly below LT1. If a long race is your goal, as a rule of thumb make the last 30–40 minutes of every second or third long ride steady, between the two thresholds: breathing deeper, sentences getting short.
• Sunday: long easy run of 60–75 minutes below LT1.

Have more hours? Add easy time, not more hard sessions.

Most sessions here are easy, as in successful endurance athletes: about four in five of their sessions are done at low intensity, near or below LT1 (8). They train far more than most amateurs, so treat that as a direction, not a target.

Keeping long sessions easy: cap them by power or pace, and accept a few beats of heart-rate drift late on. Heart rate drifts a little even below LT1. If it climbs a lot at the same power or pace over a steady hour (called decoupling), first rule out heat, poor fuelling and starting tired; if none applies and it happens on more than one long session, you were probably above LT1, so lower your cap a little. This is a coaching rule of thumb with little research behind it.

When an LT2 session fails: stop the set, finish the time easy, and move on; one failure is often fatigue, heat or poor sleep. If two or three fail in a row, take a few easy days first. If sessions still fail once you are fresh, your number is probably too high, so retest when rested.

How to estimate both thresholds: a recommended field protocol

You need a watch and, on the bike, ideally a power meter.

Who should not test without advice: if you have a heart condition, are coming back from illness or injury, or are new to hard efforts, see a doctor first, and build a few weeks of regular training before any maximal test. Stop at once if you feel chest pain, dizziness, palpitations or breathlessness out of proportion to the effort.

1. Test your upper threshold, rested. Warm up for 15–20 minutes easy with two or three short pick-ups, then do one of these:

   • Bike: ride 20 minutes as hard as you can hold evenly. About 95% of your average power is your FTP, a rule of thumb from Allen and Coggan; the right factor varies between riders, and their protocol includes a hard 5-minute effort first, so without it treat the result as slightly high (9).
   • Run: go 30 minutes solo on a flat route, as hard as you can hold evenly. Your average heart rate over the last 20 minutes is your LTHR (2). Your average pace is an upper estimate of threshold pace, since a 30-minute effort runs slightly faster than one you could hold for an hour.

   Cool down for 10–15 minutes.

2. Find your lower threshold with a talk test, a few days later. As a practical guide: on a trainer, treadmill or flat route, start well below your usual easy pace, at an effort that feels very easy, and raise it every 5 minutes, by about 10–15 watts on the bike or 10–15 seconds per kilometre faster on the run. In the last 30 seconds of each step, read the same short paragraph aloud. Note your average heart rate and power or pace for the first step where you can read it, but not comfortably. That is your LT1 estimate (3). Then stop and cool down. If even the first step was uncomfortable to talk at, start lower next time: your easy pace has probably drifted too high.

3. Repeat the talk test once on another day. If the two results are close (within about 5 beats per minute or 10 watts), use the average. If not, use the lower one.

4. Set your easy ceiling just below your LT1 estimate. For example, if your LT1 heart rate is 142, cap easy days at about 138–140.

5. Retest both every 8–12 weeks, or sooner if sessions keep feeling much easier or harder than they should.

Not sure of any numbers? As a rough guide, use effort alone: below LT1 you can chat; between the two, full sentences get difficult; around LT2 you manage short phrases; well above it, single words.

Other options

A lab test. An incremental test with blood lactate or breathing-gas analysis gives both thresholds in one session. Ask how the thresholds are chosen: different methods give different answers from the same samples (4). Reviews of training often use fixed values of 2 and 4 mmol/L as stand-ins for LT1 and LT2 (10); your own thresholds can sit above or below them.

Critical power or critical speed. These are worked out from several all-out efforts of different lengths, and give a well-founded estimate of the upper boundary (5). Critical power usually sits above MLSS (11, 6), so if you use it for zones, set threshold intervals below it rather than at it. App estimates are only as good as the all-out efforts they come from.

What if your numbers don't match how races go?

Numbers are estimates; race results are the truth. If you blow up at a pace your threshold says you can hold, there are three common reasons:

• The test was on a good day. Retest, and trust the result that keeps repeating.
• Heat or dehydration raised your heart rate and lowered the power or pace you can sustain. In the heat, lower your targets and let feel and heart rate tell you by how much.
• You faded late in a long race. Your thresholds themselves can fall as you tire: one review reports that critical power may fall by an average of about 10% after two hours of cycling between the two thresholds, with individual changes from under 1% to about 32% (12). That, along with fuelling and heat, is why long-race pacing sits well below LT2, and why durability (how well you hold your numbers late in a long session) is worth training with long sessions.

The science behind it

Optional reading: why the labels don't line up neatly.

Two broad frameworks describe the same territory. The lactate framework, reviewed by Faude and colleagues, describes an "aerobic–anaerobic transition" with two breakpoints: the first rise in lactate above baseline and the maximal lactate steady state (1). Physiologists also describe three intensity "domains", moderate, heavy and severe, separated by two boundaries (7).

How well the methods agree depends on which boundary you mean. A review of intensity methods concluded there is reasonable support for LT1 and the first ventilatory threshold marking the moderate–heavy boundary. For the heavy–severe boundary, it found little support for most commonly used methods, with the exception of critical power and critical speed (7). In a study of 14 trained male cyclists, the first lactate threshold matched VT1; a lactate 0.5 mmol/L above it matched MLSS, while one 2 mmol/L above it matched VT2. Most lactate methods were repeatable to within about 3–4%, but the "Dmax" method varied by about 10% from test to test (4).

The upper boundary has a further wrinkle. MLSS is measured with several 30-minute constant efforts and usually falls slightly below critical power. Jones and colleagues argue that critical power is the true boundary. Exercise at it is often reported to last only about 20–30 minutes, but they note that this varies widely, from roughly 15 to 40 minutes and occasionally longer, partly because every estimate of critical power carries some error (11). A meta-analysis of nine small studies found that most common thresholds correlated with critical power but differed from it: MLSS sat on average about 11% below and VT1 about 30% below, while VT2 sat about 21% above and did not correlate significantly (6). A review by Poole and colleagues presents critical power as a coherent framework for that boundary (5). These papers come largely from researchers who champion critical power; others still treat MLSS or lactate thresholds as the practical reference, so the debate is open.

References

1. Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they?. Sports Med. 2009. doi:10.2165/00007256-200939060-00003
2. Friel J. The Triathlete's Training Bible. 4th ed. VeloPress; 2016.
3. Bok D, Rakovac M, Foster C. An Examination and Critique of Subjective Methods to Determine Exercise Intensity: The Talk Test, Feeling Scale, and Rating of Perceived Exertion. Sports Med. 2022. doi:10.1007/s40279-022-01690-3
4. Pallarés JG, Morán-Navarro R, Ortega JF, et al. Validity and Reliability of Ventilatory and Blood Lactate Thresholds in Well-Trained Cyclists. PLoS One. 2016. doi:10.1371/journal.pone.0163389
5. Poole DC, Burnley M, Vanhatalo A, et al. Critical Power: An Important Fatigue Threshold in Exercise Physiology. Med Sci Sports Exerc. 2016. doi:10.1249/mss.0000000000000939
6. Galán-Rioja MÁ, González-Mohíno F, Poole DC, et al. Relative Proximity of Critical Power and Metabolic/Ventilatory Thresholds: Systematic Review and Meta-Analysis. Sports Med. 2020. doi:10.1007/s40279-020-01314-8
7. Jamnick NA, Pettitt RW, Granata C, et al. An Examination and Critique of Current Methods to Determine Exercise Intensity. Sports Med. 2020. doi:10.1007/s40279-020-01322-8
8. Seiler S. What is best practice for training intensity and duration distribution in endurance athletes?. Int J Sports Physiol Perform. 2010. doi:10.1123/ijspp.5.3.276
9. Allen H, Coggan AR, McGregor S. Training and Racing with a Power Meter. 3rd ed. VeloPress; 2019.
10. Casado A, González-Mohíno F, González-Ravé JM, et al. Training Periodization, Methods, Intensity Distribution, and Volume in Highly Trained and Elite Distance Runners: A Systematic Review. Int J Sports Physiol Perform. 2022. doi:10.1123/ijspp.2021-0435
11. Jones AM, Burnley M, Black MI, et al. The maximal metabolic steady state: redefining the 'gold standard'. Physiol Rep. 2019. doi:10.14814/phy2.14098
12. Jones AM. The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance. J Physiol. 2024. doi:10.1113/jp284205