Lactate threshold explained: LT1, LT2 and what they mean for your zones

Almost every training plan you will ever follow is built on two invisible lines. Below the first, you can ride more or less indefinitely. Above the second, a clock starts. Everything in between is the interesting, awkward middle. Those lines are commonly called LT1 and LT2, and understanding them makes the whole architecture of zones stop feeling arbitrary.

This article covers what a lactate test actually measures, why lactate is not the villain it was cast as for fifty years, and how the two thresholds map onto the three- and five-zone models you already use.

First: lactate is a fuel, not a waste product

This is the single biggest correction to make. For decades lactate was taught as a metabolic dead-end — the sludge that built up and made your legs burn. That account has been steadily dismantled. Lactate is a substrate: it is produced continuously, even at rest, and it is shuttled between muscle fibres, from muscle to heart, and to the liver where it can be turned back into glucose. Contemporary reviews describe it as a fuel, a signalling molecule and a carbon carrier all at once.

The claim that lactate production causes exercise acidosis has also been challenged head-on in the physiology literature, with the argument that the protons come from ATP turnover rather than from lactate formation. It is worth saying plainly that this is a contested position rather than a closed case — but the practical upshot is uncontroversial: lactate is a marker of what is happening, not the thing hurting you.

The two lines, and the three domains between them

Physiologists usually describe intensity in three domains rather than in zones. The thresholds are the borders.

LT1 — the aerobic threshold, also called the first lactate turn point — is where blood lactate first lifts meaningfully above resting values. Below it, you are in the moderate domain and everything is comfortably steady. This is the ceiling of what most people mean by "Zone 2".

LT2 — variously the anaerobic threshold, maximal lactate steady state (MLSS), or in a related framing critical power — is the highest intensity at which lactate production and clearance can still find an equilibrium. Above it, nothing settles, and the effort has a finite duration written into it from the first pedal stroke.

MLSS, critical power, FTP — are these the same line?

Roughly the same neighbourhood, arrived at by different roads, and the differences have generated a genuinely lively literature. MLSS is defined from blood lactate behaviour over a set of constant-work bouts. Critical power comes from the mathematics of how long you can hold different power outputs. FTP is a practical field estimate.

Work comparing them has argued that MLSS, as conventionally determined, tends to sit somewhat below the true boundary of the heavy domain, while critical power sits at or nearer it. More recent analysis in cyclists suggests that when both are determined with strict, standardised criteria, the two converge closely — and that much of the historical disagreement was protocol noise rather than physiology. For a rider, the honest summary is: they are estimates of the same physiological boundary, they will not give you identical watts, and the number matters less than knowing which side of it you are riding on.

How reproducible is any of this?

Worth knowing before you spend money on a lab test. In well-trained cyclists, ventilatory thresholds have been reported with coefficients of variation of roughly 1.6–3.5%, and blood lactate thresholds around 3.0–3.7% — respectable. But the same study found one popular calculation method, Dmax, far less reliable, at around 10%. And a broader critique of intensity-determination methods concluded there is no single validated gold standard across the many ways of drawing these lines.

This is also why fixed lactate values should be treated as conventions rather than facts. You will see 2 mmol/L quoted for LT1 and 4 mmol/L for LT2 — the latter as "OBLA". They are useful rules of thumb and they are also arbitrary: different methods applied to the same test data do not agree with each other, and a rider's actual turn points may sit well away from the round numbers.

Mapping the thresholds onto your zones

Here is the translation, which is the part you will actually use.

Two things follow from that table. First, the popular advice to spend most of your time "easy" is really advice to spend it below LT1 — and the only way to know where that is, is to have some estimate of it. Second, the heavy domain is not forbidden; it is just expensive relative to what it returns, which is why it should be deliberate rather than accidental.

Three things people still get wrong

• Lactate causes the burn. The burning sensation and the fatigue come from a cluster of things happening at high intensity. Lactate is produced alongside them, not the cause of them — and the classic biochemical account of lactate producing the acid has been directly challenged in the literature.
• Lactate causes next-day soreness. It does not. Blood lactate is back to baseline within an hour or so of finishing. Delayed soreness is a different process entirely.
• Lactate is metabolic waste. It is a fuel. It is shuttled between fibres, between muscles, to the heart and to the liver, and oxidised or recycled into glucose.

Finding your lines without a lab

• LT2 in the field. A long, hard, evenly paced effort — 20 to 30 minutes — gives you a usable estimate in power or heart rate. Not perfect, but repeatable, which matters more.
• LT1 by breathing. The point where your breathing first becomes noticeably deliberate, and holding conversation shifts from effortless to slightly managed, is a decent practical marker.
• Test in the same conditions. Heat, fatigue and even time of day move these numbers. A comparison is only meaningful against a similar day.
• Retest seasonally, not weekly. The measurement error on all of these methods is larger than a fortnight's genuine progress.

In practice most riders never do a lactate test, and do not need one. Moveee estimates your threshold from your own riding through the power profile and FTP estimate, which gives you the LT2-ish line that sets your zones — and from there, deciding whether today sits above or below it is most of the skill. If you want the practical version of riding below LT1, our guide to Zone 2 on a smart trainer is the place to start.