LT1, LT2, and LTHR: The Lactate Thresholds Explained
LT1, LT2, and LTHR all describe the same thing from different angles: where your body crosses from easy to hard. LT1 is the lower lactate threshold (the aerobic threshold), where blood lactate first rises above its resting baseline. LT2 is the upper lactate threshold (the anaerobic threshold), where lactate starts climbing steeply and you can no longer hold a steady state. LTHR is not a third threshold at all. It's your heart rate at LT2, the single number most heart-rate zone systems are built on. This article explains what's happening underneath those labels and untangles the jargon so the terms stop confusing you.
What's actually happening: the lactate curve
Your muscles produce lactate all the time, even at rest. For decades it was blamed for the burn and fatigue of hard efforts, but that picture is outdated. Lactate is a fuel: your body shuttles it from working muscle to the heart, liver, and other muscle fibres and burns it for energy. This is the "lactate shuttle" described by physiologist George Brooks. The blood lactate you measure isn't waste; it's the balance between how fast you produce it and how fast you clear and reuse it.
That balance is what the curve below tracks. Plot blood lactate against exercise intensity and you don't get a straight line. You get two bends:
At low intensity, lactate stays flat and low. You make it and clear it at the same rate. At the first bend, LT1, production starts to edge ahead and lactate lifts measurably above baseline. Between the two bends it rises gradually but stays controllable. At the second bend, LT2, the brakes come off: clearance is maxed out, lactate accelerates, and the clock starts ticking on how long you can hold the effort. Those two bends are your two thresholds.
What is LT1 (the aerobic threshold)?
LT1, the aerobic threshold, is the intensity where blood lactate first rises clearly above its resting level. It's often pegged at roughly 2 mmol/L, though the honest definition is "the first sustained rise above baseline" rather than a fixed number. Below LT1 you're burning a high proportion of fat, breathing easily, and you could keep going for hours. It marks the top of genuinely easy training: the ceiling of Zone 2.
This is the threshold that matters for base building and the reason Zone 2 training gets so much attention: staying below LT1 lets you accumulate large volumes of aerobic work without digging a recovery hole. On the breathing side, LT1 lines up closely with the first ventilatory threshold (VT1), the point where your breathing first deepens and the "talk test" starts to get slightly harder.
What is LT2 (the anaerobic threshold)?
LT2, the anaerobic threshold, is the higher, more famous landmark: the hardest intensity you can sustain in a metabolic steady state, roughly the effort you could hold for about an hour. Push above it and lactate rises continuously until you have to back off. This is the threshold most field tests and FTP estimates are chasing, and it lines up with the second ventilatory threshold (VT2), where breathing becomes labored.
You'll constantly see LT2 written as "4 mmol/L." That number comes from Mader's classic OBLA (onset of blood lactate accumulation) work and it's a useful population average, but it is not your personal value. The intensity you can truly hold in a steady state is called the maximal lactate steady state (MLSS), and measured MLSS lactate concentrations range from about 2 to 8 mmol/L across individuals depending on training status. A fixed 4 mmol/L cutoff overestimates the real threshold for some athletes and underestimates it for others. Treat "4 mmol/L" as a sketch, not a verdict.
What is LTHR, and how is it different?
LTHR (lactate-threshold heart rate) is the heart rate that occurs at LT2. It isn't a separate physiological event; it's just LT2 expressed in beats per minute instead of pace, power, or millimoles. It matters because heart rate is the one threshold marker almost everyone can measure for free, so LTHR became the anchor for the most widely used heart-rate zone systems, including Joe Friel's.
The standard way to estimate it is Friel's 30-minute solo time trial. One detail trips people up: your LTHR is the average heart rate of the final 20 minutes of that test. You press lap at the 10-minute mark and ignore the opening 10 minutes, because heart rate lags effort early on and including it drags the average down. (You'll sometimes see it described as "averaging your heart rate at the 10-, 20-, and 30-minute marks." That's a looser approximation; the final-20-minute average is the method Friel actually publishes.) For the full protocol plus pace and power alternatives, see how to find your lactate threshold.
LT, AT, OBLA, MLSS, VT: a quick disambiguation
The science is simple; the vocabulary is a mess, because different labs and traditions named the same two landmarks differently. Here's the map:
- LT1 = aerobic threshold (AeT) = VT1 = the "first lactate turn point." The lower landmark, top of easy. Often labelled around 2 mmol/L.
- LT2 = anaerobic threshold (AnT) = lactate threshold (LT) = VT2 ≈ OBLA ≈ MLSS ≈ FTP (on the bike). The upper landmark, your ~1-hour ceiling. Often labelled around 4 mmol/L.
- LTHR = the heart rate at LT2. A number, not a threshold: the bpm you read off the upper landmark.
- OBLA is specifically the fixed 4 mmol/L definition of LT2; MLSS is the individually measured version; VT1/VT2 are the same two landmarks seen through breathing instead of blood.
When someone says "my threshold," they almost always mean LT2. When a coach talks about keeping easy days easy, they mean staying under LT1.
How the two thresholds define your training
LT1 and LT2 carve your intensity range into the three zones modern endurance coaching is built around, an idea most associated with researcher Stephen Seiler:
- Below LT1: easy aerobic work. The bulk of a well-structured week.
- Between LT1 and LT2: the "gray zone." Useful in measured doses (tempo, sweet spot), overused by most amateurs.
- At or above LT2: hard, threshold-and-above work that drives the high-end adaptations.
This three-zone view is the backbone of polarized training: lots of work below LT1, a meaningful chunk above LT2, and relatively little marooned in the middle. Once you've estimated your LTHR, you can turn it into a full set of heart-rate bands here:
Lactate-threshold HR
165bpm
cycling LTHR
| Zone | Range | What it trains |
|---|---|---|
| Z1 | Active Recovery ≤ 132 bpm | Recovery spins; flushes the legs. RPE 1–2, very easy. |
| Z2 | Endurance 133–147 bpm | Aerobic base and fat metabolism. RPE 3–4, all-day pace. |
| Z3 | Tempo 148–153 bpm | Aerobic endurance, 'sweet' steady work. RPE 5–6, comfortably hard. |
| Z4 | Threshold 154–163 bpm | Lactate threshold; sustainable hard. RPE 7–8, 20–60 min limit. |
| Z5a | VO₂max 164–168 bpm | Maximal aerobic power. RPE 9, 3–8 min efforts. |
| Z5b | Anaerobic Capacity 169–175 bpm | Anaerobic capacity, short and sharp. RPE 9–10, 30s–3min. |
| Z5c | Neuromuscular ≥ 176 bpm | Sprint power and neuromuscular drive. RPE 10, all-out. |
Z1 · Active Recovery. Recovery spins; flushes the legs. RPE 1–2, very easy.
Z2 · Endurance. Aerobic base and fat metabolism. RPE 3–4, all-day pace.
Z3 · Tempo. Aerobic endurance, 'sweet' steady work. RPE 5–6, comfortably hard.
Z4 · Threshold. Lactate threshold; sustainable hard. RPE 7–8, 20–60 min limit.
Z5a · VO₂max. Maximal aerobic power. RPE 9, 3–8 min efforts.
Z5b · Anaerobic Capacity. Anaerobic capacity, short and sharp. RPE 9–10, 30s–3min.
Z5c · Neuromuscular. Sprint power and neuromuscular drive. RPE 10, all-out.
- LTHR-based zones are more individual than %HRmax because they anchor to your actual threshold, not an age estimate. Cycling and running LTHR differ, so test each sport separately.
- Field test: warm up, then ride or run a hard 30-minute solo time trial. Your LTHR is the average heart rate of the final 20 minutes (Friel's protocol).
A PDF of your personalized results, with a QR code to reopen them anytime.
The honest caveats
The fixed millimole numbers are averages, not laws. Your real LT1 and LT2 sit where your curve bends, which only a graded lab test measures directly. Field estimates like the 30-minute test get most athletes close enough to train well, but they assume even pacing and a genuine maximal effort. And thresholds move: the entire point of training is to push LT2 to a higher intensity, so retest every 4 to 8 weeks rather than trusting a number from last season. Treat your thresholds as working estimates you refine, not permanent labels.
Sources
The two-threshold (LT1/LT2) framework and the three-zone training model are most associated with Stephen Seiler's research on intensity distribution. George Brooks developed the lactate-shuttle concept that reframed lactate as a fuel rather than waste. The fixed 4 mmol/L "OBLA" definition of the anaerobic threshold originates with Mader and colleagues; subsequent maximal-lactate-steady-state (MLSS) research shows individual threshold lactate concentrations vary widely (roughly 2–8 mmol/L), which is why a single cutoff misclassifies many athletes. Joe Friel describes the 30-minute time-trial protocol for estimating lactate-threshold heart rate and the LTHR-based zones derived from it.