What is lactate threshold?

The intensity above which lactate accumulates in the blood faster than you can clear it. It is the single best physiological predictor of endurance race pace — better than VO₂max.

Lactate is produced continuously, at rest and at every running speed, and it is continuously consumed as fuel by other tissues. The threshold is not where production begins — that is a persistent myth — it is where production outruns clearance and blood concentration starts to climb. Below it, you are in a steady state you could hold for a long time. Above it, a clock starts.

Two thresholds are usually distinguished. The first, LT1, is where blood lactate first rises above resting baseline, around 65–75% of heart-rate reserve for most runners. The second, LT2, is where the rise becomes steep and unsustainable, around 80–88% of heart-rate reserve. LT2 is the one people usually mean by "threshold pace", and it approximates the fastest pace you could hold for roughly an hour.

Threshold matters more than VO₂max for distance racing because it determines what fraction of your aerobic ceiling you can actually use. Two runners with identical VO₂max values can race very differently if one sustains 88% of it and the other 75%. It is also more trainable than VO₂max in already-trained athletes, which is why threshold work is the backbone of most marathon builds.

For a marathon, the relevant fact is that race pace sits below LT2 — typically well below it for recreational runners. The marathon is not run at threshold; threshold training raises the ceiling that marathon pace sits under.

Glossary: LT1 (Lactate Threshold 1), LT2 (Lactate Threshold 2)