Training science
Lactate threshold: the most important number in distance running?
Runners obsess over VO2max, but the number that more often separates two athletes with the same engine is the pace they can hold before lactate runs away from them. Here is what lactate threshold really is, and how to find yours.
For decades, runners have been taught that lactic acid is the enemy: the toxic byproduct that floods the muscles, makes them burn, and forces you to slow down. It is one of the most durable myths in endurance sport, and it is wrong. Lactate is not a waste product. It is a fuel, and the rate at which your body produces and clears it tells you more about your distance running potential than almost any other measurement.
This article looks at what lactate actually is, what the first and second lactate thresholds (LT1 and LT2) really mean, why your threshold predicts race performance better than VO2max alone, how to estimate it without a laboratory, and how to train it.
Lactate is a fuel, not a poison
When you break down carbohydrate for energy, the end product of glycolysis is pyruvate. During hard exercise, some of that pyruvate is converted to lactate. Far from being a dead end, lactate is then shuttled around the body and burned for energy by your muscle fibres, your heart and even your brain. This is the lactate shuttle, and it runs constantly, including at rest.
The burn you feel in a hard interval is not lactate scalding your muscles. It is the broader chemical disturbance of intense work, including a rise in acidity that comes largely from other reactions in the cell. Lactate itself is closely associated with fatigue because it rises when you are working hard, but association is not cause. Treat lactate as a signal on the dashboard, not the thing breaking the engine.
LT1 and LT2: the two thresholds that matter
There is not one lactate threshold but two useful boundaries, and confusing them is the source of endless argument. Faude, Kindermann and Meyer (2009) reviewed the tangle of competing definitions and laid out the framework most coaches now use: the transition from aerobic to anaerobic running, marked by a first and a second threshold.
LT1, the first (aerobic) threshold.
The lowest intensity at which blood lactate climbs just above its resting baseline, typically around 2 mmol/L. Below LT1 you can run almost indefinitely. This boundary is the top of true easy running and the anchor for Zone 2 training.
LT2, the second (anaerobic) threshold.
The highest intensity at which lactate production and clearance still balance, so the concentration holds steady rather than spiralling upward. This is the maximal lactate steady state, often near 4 mmol/L. It is the wall between sustainable and unsustainable effort, and it is what most people mean by “lactate threshold” or “anaerobic threshold.”
Billat and colleagues (2003) describe LT2 as the maximal lactate steady state and report that it sits remarkably close to real race pace: the average velocity over a marathon is just below it. That is the practical power of the concept. Your second threshold is not an abstraction; it is roughly the pace you can race for an hour or more.
Why threshold beats VO2max for predicting race times
VO2max measures the size of your aerobic engine, the maximum rate at which you can consume oxygen. It matters, and Bassett and Howley (2000) make the case that oxygen delivery is the primary limit on it. But two runners with identical VO2max values can post very different race times, because performance depends on what fraction of that maximum you can actually sustain.
Joyner and Coyle (2008), in their landmark review of the physiology of champions, frame endurance performance as the product of three factors: VO2max, the lactate threshold, and running economy. VO2max and threshold then interact to set your “performance VO2,” the oxygen uptake you can hold for a given duration. A high ceiling is wasted if your threshold sits at 70 percent of it.
The classic demonstration is Coyle and colleagues (1988). Studying cyclists matched for VO2max, they found that those whose lactate threshold occurred at a higher percentage of VO2max lasted more than twice as long to fatigue at the same hard workload. Over 92 percent of the variance in endurance was explained by the threshold fraction and muscle capillary density, not by VO2max. The pattern repeats across endurance sports: among athletes of similar maximal capacity, the one with the higher threshold usually wins.
How to estimate your threshold without a lab
A proper lactate test means pricking a fingertip at rising speeds and plotting the curve. Most runners will never do one, and they do not need to. Three field methods get you close enough to train by.
1. The 30 to 60 minute time trial.
Warm up, then run as evenly and hard as you can sustain for 30 minutes solo. Your average pace over that effort, and the average heart rate over the final 20 minutes, sit very close to your LT2. This is the most reliable estimate you can make on your own and gives you both a threshold pace and a lactate threshold heart rate to train from.
2. The talk test.
Persinger and Foster (2004) showed that the point at which you can no longer speak comfortably in full sentences tracks the ventilatory threshold, which sits near your lactate threshold. If you can chat freely you are below LT1; if you can manage only a few words at a time you are above LT2. The grey zone in between is roughly threshold effort.
The third method is critical speed. Run two or three maximal efforts of different durations, for example a hard 3 minutes and a hard 12 minutes, and the speed your performance settles toward over longer durations approximates the boundary between sustainable and unsustainable running. Critical speed is closely related to the maximal lactate steady state and is a sound lab free proxy for LT2.
However you estimate it, anchor the result to effort and breathing rather than a single heart rate number, which drifts with heat, fatigue and the length of the run. For more on why fixed percentages mislead, see our guide to heart rate zones for running.
Training the threshold: tempo runs and beyond
The good news is that lactate threshold is highly trainable, arguably more so than VO2max in experienced runners. Jones and Carter (2000) reviewed how endurance training reshapes aerobic fitness and noted that the velocity at lactate threshold improves with training even when VO2max has plateaued, driven by greater mitochondrial density, capillarisation and a rightward shift in the lactate curve.
The staple session is the tempo run: a sustained effort at or just below LT2, the comfortably hard pace you could race for about an hour. Twenty to forty minutes of continuous tempo, or broken into longer cruise intervals such as three to five repeats of eight to ten minutes with short recoveries, teaches the body to clear lactate at higher speeds. Run it controlled, not heroic; threshold work that tips into a race effort defeats the purpose.
Threshold sessions sit on top of a deep aerobic base, not in place of it. The large volume of easy running below LT1 is what builds the machinery that makes a high threshold possible, while a smaller slice of genuinely hard work lifts the ceiling that threshold pushes against. If you want to raise that ceiling too, our guide to VO2max for runners covers the hard end of the spectrum.
So is it the most important number?
Not on its own. Endurance is a system: a big VO2max sets the ceiling, running economy decides how cheaply you move, and the lactate threshold decides how much of that ceiling you can actually use over race distance. But if you had to pick one number that best separates runners of similar talent and predicts how they will perform on the day, the fraction of VO2max you can hold at threshold is the strongest candidate. It is the number that turns a big engine into a fast race.
Frequently asked questions
What is lactate threshold in running?
Lactate threshold is the running intensity above which blood lactate begins to accumulate faster than your body can clear it. Below it, lactate production and removal stay balanced and you can hold the pace for a long time. Above it, lactate and fatigue rise steadily, so the pace is unsustainable. It is one of the strongest predictors of distance running performance.
Is lactic acid what makes my legs burn and tired?
No. Lactate is not a waste product and does not cause the burn directly. It is a fuel that muscles, the heart and the brain readily use for energy. The burning sensation and fatigue come from the broader metabolic disturbance of hard exercise, including rising acidity from other sources, not from lactate itself.
What is the difference between LT1 and LT2?
LT1, the first or aerobic threshold, is the lowest intensity at which lactate rises just above resting levels, around 2 mmol/L. LT2, the second or anaerobic threshold, is the highest intensity you can hold with lactate stable, often near 4 mmol/L. Easy running sits below LT1; threshold or tempo running sits at or just below LT2.
How do I find my lactate threshold without a lab test?
Use a 30 minute time trial: your average pace and heart rate over a hard, even 30 minute solo effort sit close to your threshold. The talk test also works, as the point where you can no longer speak in full sentences tracks the ventilatory threshold closely. Critical speed from two or three maximal efforts is another lab free estimate.
What pace should a tempo run be?
A classic tempo run is performed at or just below lactate threshold, a comfortably hard effort you could sustain for roughly an hour in a race. It should feel controlled but not easy, around 25 to 30 seconds per kilometre slower than 5 km race pace for many runners. You should manage only short phrases, not full conversation.
Related reading: Zone 2 training for runners: what the research actually shows.
References
- Faude, O., Kindermann, W. and Meyer, T. (2009) ‘Lactate threshold concepts: how valid are they?’, Sports Medicine, 39(6), pp. 469 to 490. PubMed.
- Joyner, M.J. and Coyle, E.F. (2008) ‘Endurance exercise performance: the physiology of champions’, The Journal of Physiology, 586(1), pp. 35 to 44. PubMed.
- Bassett, D.R. and Howley, E.T. (2000) ‘Limiting factors for maximum oxygen uptake and determinants of endurance performance’, Medicine & Science in Sports & Exercise, 32(1), pp. 70 to 84. PubMed.
- Coyle, E.F., Coggan, A.R., Hopper, M.K. and Walters, T.J. (1988) ‘Determinants of endurance in well-trained cyclists’, Journal of Applied Physiology, 64(6), pp. 2622 to 2630. PubMed.
- Jones, A.M. and Carter, H. (2000) ‘The effect of endurance training on parameters of aerobic fitness’, Sports Medicine, 29(6), pp. 373 to 386. PubMed.
- Billat, V.L., Sirvent, P., Py, G., Koralsztein, J.P. and Mercier, J. (2003) ‘The concept of maximal lactate steady state: a bridge between biochemistry, physiology and sport science’, Sports Medicine, 33(6), pp. 407 to 426. PubMed.
- Persinger, R., Foster, C., Gibson, M., Fater, D.C.W. and Porcari, J.P. (2004) ‘Consistency of the talk test for exercise prescription’, Medicine & Science in Sports & Exercise, 36(9), pp. 1632 to 1636. PubMed.
All citations point to peer reviewed primary sources. Page numbers and volume details are presented per Harvard referencing convention.
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