Lactate Threshold

Two thresholds get talked about. LT1, the aerobic threshold, is the first gentle rise in blood lactate above resting levels, roughly where fat oxidation starts giving way to carbohydrate. It is an all-day intensity. LT2, the anaerobic or lactate threshold proper, sometimes called OBLA for onset of blood lactate accumulation, is the sharper inflection where production outruns clearance. That one holds for 30 to 60 minutes at most.

The gold standard is a graded lab test with blood lactate sampled at each stage, marking LT1 near 2 mmol/L and LT2 near 4 mmol/L. Those markers come out of work by Sjödin and Jacobs, and Skinner and McLellan's widely cited three-phase model of exercise intensity domains. Field substitutes trade precision for access, since most runners cannot get near lab lactate testing: the average heart rate from a 30-minute time trial as a proxy for LTHR, or the Conconi test.

Both thresholds are expressed relative to your own max heart rate or pace, so there is no universal LT2 number. As a rough heart-rate heuristic, LT2 in trained endurance athletes commonly falls around 85 to 92% of max heart rate, with LT1 lower, often cited around 70 to 80%. Both figures shift with training history and with the testing method used.

LT1 and LT2 mark the borders of the three-zone intensity model, easy, moderate and hard, that modern zone systems including polarized and pyramidal models are built on. Threshold pace is a common prescription anchor: tempo runs and cruise intervals usually sit at or near LT2. Improvement in LT2 pace across a block is also one of the more direct fitness signals available without racing.

Fixing your threshold heart rate with a generic formula, 85% of max and done, ignores that the real threshold moves as you get fitter and sits in different places for two runners with the same max heart rate. The other trap is living at LT2. Training near threshold too often, the classic gray zone, buys disproportionate fatigue for the aerobic return, compared with spending that time either clearly easy or clearly hard.