When a coach says "train at your lactate threshold," they are describing a zone without specifying which of three distinct metabolic boundaries they mean.
This is not a semantic problem. LT1, LT2, and MLSS trigger different physiological adaptations and respond to different training prescriptions. Conflating them systematically misallocates training effort.
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The Three Thresholds Defined
LT1 (First Lactate Threshold / Aerobic Threshold): The intensity at which blood lactate first rises measurably above resting baseline — typically 1.5–2.0 mmol/L. Below LT1, exercise is fuelled almost entirely by aerobic metabolism. LT1 corresponds closely to the first ventilatory threshold (VT1) and is the boundary between Zone 1 and Zone 2 training.
This is the threshold that low-intensity, high-volume base training is designed to raise. Improving LT1 means you can sustain higher absolute workloads with minimal lactate accumulation — the foundation of endurance capacity.
LT2 (Second Lactate Threshold / Anaerobic Threshold): The intensity at which lactate production begins to exceed clearance, producing an accelerating rise. Typically 4–6 mmol/L. This corresponds to the second ventilatory threshold (VT2) and approximates FTP in cycling and marathon pace in running for well-trained athletes.
LT2 is what most coaches mean when they say "threshold training." Tempo runs, sweet spot work, and 20-minute FTP tests are all targeting this zone.
MLSS (Maximal Lactate Steady State): The highest intensity at which blood lactate can be maintained at a constant concentration over time — typically defined as no increase exceeding 1 mmol/L across the final 20 minutes of a 30-minute constant workload.
MLSS is not identical to LT2. Research consistently shows MLSS occurs at 0.5–1.5 mmol/L lower lactate than LT2 in trained athletes. Athletes training "at threshold" based on LT2 estimates are frequently training above their MLSS — which is physiologically sustainable for minutes, not hours.
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Why the 4 mmol/L Fixed Criterion Is Wrong for Most Athletes
In 1976, Mader et al. proposed 4 mmol/L as a universal threshold marker. This entered coaching practice as "the" threshold value.
The problem: lactate response is highly individual. In trained athletes, LT2 commonly sits at 3–7 mmol/L depending on genetics, training history, and testing conditions. A fixed 4 mmol/L criterion misidentifies the threshold for the majority of athletes — overestimating it in some, underestimating it in others.
More accurate methods:
- D-max method: identifies the point of maximum perpendicular distance from a line connecting the first and last lactate values on the curve — individual to each athlete
- Individual anaerobic threshold (IAT): based on the slope of the lactate curve rather than an absolute value
- MLSS confirmation: a 30-minute constant-load test at estimated MLSS intensity to confirm the ceiling of aerobic-dominant exercise
Matching Training to the Right Threshold
The adaptive target determines which threshold to train at:
| Adaptation Goal | Target Threshold | Protocol |
|---|---|---|
| Expand aerobic base | Below LT1 | Long slow distance, Zone 1–2 |
| Raise LT1 absolute workload | Just above LT1 | High-volume Zone 2, weekly hours |
| Raise MLSS / FTP | At or slightly above MLSS | 20–40 min efforts, sweet spot |
| Develop VO2max stimulus | Well above LT2 | 4–8 min intervals, 5K effort |
For cyclists, accurate threshold identification starts with reliable FTP estimation. The FTP calculator at winsport.uk/tools/cycling/ftp-calculator estimates FTP from time-trial results and riding data, with training zone breakdowns aligned to the LT1/LT2/MLSS framework for structured periodisation.
Are you building training zones from a threshold assessment that specifies which of the three thresholds it's measuring — or a number from a test?