If you've ever dug past the surface of endurance training, you've run into the word lactate. Usually attached to other words like threshold, clearance, accumulation, or that old chestnut, lactic acid — which is, by the way, not what you actually produce when you exercise. The physiology around lactate is genuinely important for endurance athletes, and it's also one of the most misunderstood topics in the sport. Most athletes who 'know' lactate from popular training articles are working with a simplified version that leaves out the useful parts.
This guide is the honest, practical version. What lactate actually is. What the lactate threshold actually measures. Why there are two thresholds instead of one. How LT1 and LT2 relate to FTP, Zone 2, and your day-to-day training. And how you'd actually use lactate information to train better — without needing to become a sports physiologist.
What is lactate, really?
Lactate is a molecule your muscles produce all the time, not just when you're working hard. It is not a waste product. It is not what causes the burning sensation in your legs (that's mostly hydrogen ions and potassium shifts, not lactate). And it is not what causes next-day muscle soreness (that's muscle damage, unrelated).
What lactate actually is, is a fuel. When your muscles produce lactate, they and neighbouring tissues (other muscles, the heart, the brain) can absorb and oxidize it as energy. At low intensities, the rate at which your body produces lactate is equal to the rate at which it clears it, so blood lactate levels stay low and stable (around 1 mmol/L at rest and in Zone 2). As intensity rises, production rises faster than clearance, and blood lactate concentration starts to climb. The point at which that climb begins, and the point at which it accelerates sharply, are the two lactate thresholds that matter for training.
What are LT1 and LT2? (The two lactate thresholds)
The key insight that most simplified training articles leave out is that there are two lactate thresholds, not one. They mean different things and they are useful for different reasons. The table below summarizes both at a glance.
| Threshold | Other names | Blood lactate | What it marks | FTP equivalent |
|---|---|---|---|---|
| LT1 | Aerobic threshold, first lactate turn point | ~1.5–2.0 mmol/L | Top of true Zone 2; boundary between endurance and tempo | ~70–75% of FTP |
| LT2 | Anaerobic threshold, MLSS, maximal lactate steady state | ~3.0–5.0 mmol/L (individual) | Boundary between sustainable and unsustainable efforts | ~95–100% of FTP |
The difference between LT1 and LT2 is not academic. They define the two boundaries of endurance training: LT1 is where Zone 2 ends, and LT2 is where sustainable efforts end. Most good training spends a lot of time below LT1 (Zone 2 volume) and targeted time around LT2 (threshold and sweet spot intervals).
How is lactate threshold measured?
In a lab, lactate threshold testing looks like this. You perform a graded exercise test on a bike or treadmill — starting easy and progressing through increasing intensity steps of 3 to 5 minutes each. At the end of each step, a technician takes a small drop of blood from your fingertip or earlobe and measures the lactate concentration with a portable analyzer. At the end of the test, the lab plots lactate concentration against intensity, identifies the points where lactate starts to rise above baseline (LT1) and where it starts to rise sharply (LT2), and reports the corresponding power, pace, or heart rate values.
The test is not cheap, but it is the gold standard for building accurate training zones. Athletes tested in a lab often discover their true LT2 is meaningfully different from their field-test FTP — sometimes by 10 watts or more. Those athletes have been training in zones that were slightly off for months or years, and tuning the zones to their actual lactate values produces surprisingly quick improvements in how training feels and how it responds.
Portable lactate meters exist (the Lactate Plus, EKF Biosen) and a minority of dedicated amateurs do their own testing. The protocol is the same as in a lab, but the handling and timing are particular, and most athletes who try this find the result is less reliable than just paying for one lab test a year.
How is lactate threshold different from FTP?
This is the most practically useful question, and the honest answer is that FTP is trying to estimate LT2 from field data — imperfectly, but well enough to be useful.
FTP (Functional Threshold Power) is defined as the highest average power you can sustain for approximately one hour. LT2 is defined as the highest intensity at which lactate stays in steady state. For most trained cyclists, these two numbers line up closely — the effort you can sustain for about an hour is approximately your maximum lactate steady state. That's why FTP became a practical shortcut: you can estimate LT2 from a 20-minute field test without needing a lab, a technician, or blood samples.
But FTP is an estimate, not the real thing. Two athletes with identical FTP values can have different true LT2 values, because their fibre-type distributions, lactate-clearance capacities, and pacing ability on a 20-minute test all vary. FTP field tests also sit under the direct influence of pacing skill, motivation, testing state, and testing protocol — which is why different test methods (20-minute, ramp, 8-minute) produce slightly different numbers for the same athlete. A lab lactate test, done properly, is more objective.
For most athletes, most of the time, FTP is accurate enough for training zones to work well. But if you've found that threshold intervals at what should be 95–105% of FTP feel much harder than they should, or that your planned Zone 2 heart rate is higher than you can hold without drifting upward, those are signs your FTP might not match your true LT2. Lab testing resolves the question.
How does Zone 2 relate to LT1?
The Zone 2 concept most endurance coaches talk about is effectively defined by LT1. Zone 2 is work below LT1 — low enough that lactate stays near baseline, fat oxidation is the dominant fuel, and the session produces the mitochondrial and capillary adaptations that make up the aerobic base. When Iñigo San Millán talks about Zone 2 on the Peter Attia podcast, the intensity he describes and the lactate values he cites (1.5 to 2.0 mmol/L) are exactly the LT1 definition.
This is why Zone 2 done well has a ceiling that matters. Drifting from just below LT1 up to just above LT1 moves you from productive base work into a different metabolic state — lactate starts accumulating, fat oxidation drops, glycogen use rises, and the adaptation profile changes. The 'talk test' and heart rate drift signals endurance coaches rely on for monitoring Zone 2 sessions are, underneath, crude proxies for whether you've crossed LT1.
How do you train each threshold?
Different training intensities drive different adaptations, and each threshold has its own productive training zone around it.
- Below LT1 (Zone 2) — long, easy endurance volume. This builds mitochondrial density, capillary networks, and fat oxidation capacity. It is the foundation of everything else. Most endurance plans spend 60–90% of weekly time here.
- Around LT2 (threshold intervals, sweet spot) — intervals of 10 to 40 minutes at or just below your threshold intensity. Sessions like 2 × 20 min, 3 × 15 min, or 4 × 10 min at ~95–100% of LT2 are classic. This is the work that raises LT2 itself over time, pushing your sustainable ceiling higher.
- Above LT2 (VO2max intervals) — shorter, harder intervals (3 to 8 minutes each) at intensities well above LT2. This raises VO2max, which indirectly raises LT2 by increasing the absolute power at which your lactate steady state occurs.
A well-structured training plan hits all three zones in proportion. Spending 100% of your time above LT1 grinds you into a hole with little gain. Spending 100% of your time below LT1 means you never raise your ceilings. Most amateurs under-do Zone 2 and over-do the middle.
What are the common misconceptions about lactate?
Five misunderstandings show up in almost every conversation about lactate in endurance sport.
- Lactate is not lactic acid. Lactic acid does not exist in meaningful quantities in your body during exercise. What your muscles produce is lactate, which is a different molecule and is not acidic on its own.
- Lactate does not cause the burning sensation. The burning in hard intervals is mostly caused by hydrogen ions, potassium shifts at the neuromuscular junction, and other metabolic factors. Lactate appears alongside these but does not cause them.
- Lactate is not a waste product. It is an active fuel source that your muscles, heart, and brain preferentially use at moderate intensities. The word 'waste product' is simply wrong and has been for decades.
- Lactate does not cause DOMS (next-day muscle soreness). DOMS is caused by muscle damage and the inflammatory response to it, and it peaks 24–48 hours after exercise — long after any lactate from the session has been cleared. The two are unrelated.
- There is no magic 4 mmol/L threshold. The old '4 mmol/L anaerobic threshold' was a statistical average from early research, not a universal physiological constant. Individual LT2 values cover a wide range (typically 2.5 to 6 mmol/L or more) and using the 4 mmol/L value as a hard rule produces wrong zones for most athletes.
When should you consider a lactate test?
For most amateur athletes, the practical answer is: not as a routine. A lab lactate test is useful in specific situations — but it is not required for most training to be effective.
- When your FTP-based zones feel off. If threshold intervals feel way harder than the power suggests, or your Zone 2 heart rate target feels surprisingly low or high, a lab test will reconcile the numbers.
- When you're preparing for a serious A race. A baseline lactate test at the start of a build phase gives you objective zones for the training block. A follow-up test 8–12 weeks later measures whether the block worked.
- When you hit a plateau that training changes don't solve. Sometimes the plateau is caused by training in zones that don't match your actual physiology. Resetting the zones with a lab test often unlocks progress that the athlete had given up on.
- When you're a competitive cyclist, triathlete, or runner interested in squeezing out the last few percent. At the amateur-elite level, objective zones are more valuable than estimated ones.
What lactate testing is not good for: replacing FTP field tests for athletes who can't easily access labs, or producing a single magic number that solves everything. It's a precision tool, not a panacea.
Key takeaways
- Lactate is a fuel, not a waste product. It does not cause burning, soreness, or fatigue directly.
- There are two lactate thresholds: LT1 (top of Zone 2) and LT2 (maximal lactate steady state, roughly what FTP estimates).
- LT1 marks the boundary between easy endurance work and moderate-intensity work. LT2 marks the boundary between sustainable and unsustainable efforts.
- FTP is a practical field estimate of LT2. It's usually close enough, but a lab lactate test is the gold standard.
- Zone 2 is defined by LT1 — work below this intensity for the mitochondrial, capillary, and fat-oxidation adaptations.
- Threshold intervals around LT2 are what drive LT2 itself higher over time.
- The old 4 mmol/L threshold is a statistical average, not a universal constant — individual values vary widely.
- Consider a lab test if your FTP-based zones feel off, if you're preparing for an A race, or if you've hit a plateau your training changes don't solve.
Frequently asked questions
Is FTP the same as lactate threshold?
No, but they are closely related. FTP is a practical field estimate of LT2, typically measured from a 20-minute or ramp test. LT2 is the physiological concept FTP is trying to estimate. For most trained athletes, FTP lines up closely with LT2, but not always — pacing ability, testing state, and the specific protocol used all influence FTP, while LT2 is measured directly from blood samples. When FTP and LT2 disagree, LT2 is the more objective number.
Do I need a lactate meter at home?
Probably not. Lactate meters are expensive, the strips are expensive, the protocol is particular, and the results are noisier than a lab test unless you're trained to do them. A single lab lactate test once or twice a year is more useful for most athletes than trying to maintain a home testing setup. If you're deeply interested in physiology and enjoy the process, a home meter can be a fun tool — but it's not a training necessity.
What lactate values should I see at rest, in Zone 2, and at threshold?
Rough ranges: at rest, 0.5–1.5 mmol/L. In true Zone 2 (below LT1), 1.5–2.0 mmol/L. Just below LT2, 3–4 mmol/L. At LT2 itself, typically 3.5–5 mmol/L but varies by athlete. Above LT2, rising — 6 mmol/L and above during VO2max intervals, 10 mmol/L or more at all-out short efforts. Your personal values for LT1 and LT2 are what matter, not the population averages.
Can heart rate replace lactate testing?
Partially. Heart rate is an imperfect but usable proxy for lactate threshold in most trained athletes, which is why heart-rate-based zones work at all. The challenges are that heart rate lags effort by 30–60 seconds, drifts upward over long efforts (cardiac drift), and is influenced by heat, caffeine, sleep, and stress. For most day-to-day training, heart rate zones derived from a field test are good enough. For precision work or to resolve conflicts with FTP, lactate testing remains the gold standard.
Why do I see '4 mmol/L' cited as the anaerobic threshold in so many articles?
It's a historical artefact from early lactate research in the 1970s and 80s. The original studies by Kindermann and others found that a statistical average of LT2 values in their subjects fell around 4 mmol/L, and that number became a shorthand. Individual LT2 values actually range widely — some athletes steady-state at 2.5 mmol/L, others at 6 mmol/L — and using 4 as a universal target produces wrong zones for most people. Modern lactate testing finds each athlete's own LT2 directly rather than assuming a universal value.
Does lactate threshold improve with training?
Yes, both LT1 and LT2. LT1 rises with accumulated Zone 2 volume over months and years — an athlete with a long base can sustain higher power at the same low lactate value. LT2 rises with threshold-intensity work (sweet spot and threshold intervals) and, indirectly, with improvements in VO2max. Both thresholds are trainable at any age, though the pace of improvement slows with training age. Most amateurs can meaningfully raise both LT1 and LT2 over a focused 8–12 week block.
How CoreRise uses lactate threshold information
CoreRise's training zones are built on FTP or threshold pace by default, because that's what most athletes have access to. But if you've done a lab lactate test and have objective LT1 and LT2 values, the coach can use those directly — you tell the coach 'my LT1 is 2.3 W/kg at 140 bpm, my LT2 is 3.8 W/kg at 165 bpm', and it rebuilds your zones around those anchors instead of the FTP-derived default. Every subsequent workout is written against the lactate-accurate zones, and every Zone 2 session gets evaluated against the LT1 heart rate rather than a generic percentage.
If you don't have lab data, the coach uses FTP- or threshold-pace-derived zones — which, as the article above explains, is a good approximation most of the time. But when your training stops behaving the way it should (intervals feeling wrong, Zone 2 drift, plateau that won't budge), the coach can flag that mismatch as a candidate reason and suggest a lab test as a diagnostic rather than a routine.
- Default zones are FTP or threshold-pace derived, matching how most athletes train.
- You can input lab-measured LT1 and LT2 values and the coach rebuilds zones around them.
- Zone 2 sessions are evaluated against your LT1-anchored HR and power when available.
- When training zones feel mismatched to how sessions actually go, the coach can recommend a lab test as the next diagnostic step.
- Over time, retesting and updating LT1 / LT2 feeds into a cleaner view of your physiological progress than FTP alone.
Antoine Boudet is the founder of CoreRise. He finished Ironman 70.3 Oceanside in 2026 and writes the evidence-based Learn hub articles for runners, cyclists and triathletes, drawing on the research literature and his own training.