Lactate threshold
Evidence: strong
Velocity at threshold is among the best single predictors of endurance performance. The threshold itself is a region, not a precise line, and “lactate” is a fuel, not a waste product.
The lactate threshold is the exercise intensity above which blood lactate accumulates faster than the body can clear it. The literature distinguishes a first, lower threshold (LT1, the aerobic threshold), where lactate first rises above resting levels, from a second, higher threshold (LT2), which approximates the maximal lactate steady state (Faude, Kindermann & Meyer 2009; Jones & Carter 2000).
What lactate actually is
The threshold is widely misunderstood because lactate is. Lactate is not a waste product and not the cause of fatigue or of muscle soreness. It is a fuel: it is produced continuously, shuttled between and within cells, and oxidised for energy, and the better-trained the athlete, the better they clear and reuse it (San Millán & Brooks 2018). Blood lactate rises not because the muscle has “run out of oxygen” but because, as intensity climbs, production by fast-twitch fibres outpaces the body’s capacity to clear it through the lactate-shuttle transporters. The threshold is therefore a balance point between production and clearance, and training shifts that balance, which is why it moves with fitness.
Why it predicts performance
The running velocity at threshold integrates all three classical determinants: it depends on VO₂max, on the fraction of VO₂max that can be sustained, and on running economy. In Farrell’s foundational data, velocity at the onset of lactate accumulation correlated about 0.91 with performance across race distances, the strongest of any single variable measured (Farrell et al. 1979). Within trained groups, where VO₂max values cluster, threshold velocity out-predicts VO₂max (Coyle 1995). For the marathon in particular, the pace that can be held for the distance sits close to the second threshold, which is why threshold is the determinant marathoners train hardest.
A caveat sharpens the point. The threshold expressed as a percentage of VO₂max does not cleanly separate elite from recreational runners; one analysis found it sat near 83% in all groups, while maximal aerobic speed dominated the correlation with performance (Front Physiol 2020). The robust predictor is the velocity at threshold, not the fraction of VO₂max it represents. A runner raises threshold velocity by improving any of the three underlying determinants, not by chasing a percentage.
The definitions disagree
“Lactate threshold” is not one line but a family of them, and they do not all fall at the same intensity in the same runner. The second threshold alone has several competing definitions: a fixed 4 mmol/L, historically called the onset of blood lactate accumulation (OBLA); a fixed rise above baseline; curve-fitting methods such as Dmax and modified Dmax; and individual-anaerobic-threshold methods (Faude et al. 2009). The fixed 4 mmol/L value is a useful convention, not a biological constant; individual maximal lactate steady states scatter around it, so anchoring training to a fixed 4 mmol/L misreads many runners. The genuine gold-standard criterion, the maximal lactate steady state, requires several constant-pace runs on separate days and is rarely done outside research. The practical consequence is that a “threshold pace” depends on which protocol produced it, so the threshold is best treated as a narrow region rather than a precise line. How these are measured, in the lab and with a finger-prick analyser, is covered in lactate testing.
How it is trained
Training raises the threshold by improving lactate clearance and the fraction of VO₂max that can be sustained, through more mitochondria, denser capillaries, and more lactate-transport proteins (Jones & Carter 2000). The work that drives this is sustained or broken running at or just below the second threshold, the basis of tempo runs and of double-threshold training, sitting on top of the easy volume that builds the aerobic base. Holding intensity precisely at threshold, rather than drifting above it, is the point of controlling sessions by lactate testing or by feel.
Position relative to VO₂max
Trained athletes hold threshold at roughly 80 to 90% of VO₂max; untrained people at around 50 to 60% (Joyner & Coyle 2008; Coyle 1995). Raising the fraction of VO₂max that can be sustained is one of the main adaptations endurance training produces, and one of the clearest signs that a runner’s aerobic base is developing even when VO₂max has plateaued. Blood lactate at a given pace is also sensitive to carbohydrate availability, so a depleted, low-carbohydrate state shifts the whole curve and can mislead a test; pre-test diet should be standardised.