Lactate Threshold

Lactate threshold is the exercise intensity at which lactate starts to build up in your blood faster than your body clears it. There are two of them. LT1 is where lactate first rises above resting levels, and LT2 is the highest intensity you can hold with lactate still stable, usually sustainable for 30 to 60 minutes.

Those two points set the pace of most endurance racing. In Sjödin and Jacobs's 1981 study of 18 Stockholm marathoners, running speed at a blood lactate of 4 mmol/L explained 92 percent of the variation in marathon pace. Joyner and Coyle's model of endurance performance uses three terms, and threshold is one of them. VO2 max sets your aerobic ceiling. Lactate threshold decides how much of that ceiling you can use over a long race. Economy converts the oxygen into speed.

Nobody agrees on exactly how to measure it, though. Faude, Kindermann and Meyer found 25 different lactate threshold concepts in the literature in 2009. For the wider physiology of oxygen delivery, zones and training load, read the performance physiology guide.

01Two thresholds on one lactate curve

Your muscles make lactate all the time, even at rest with plenty of oxygen available. George Brooks's review of lactate shuttle research describes it moving from the cells that produce it to the cells that use it, as a fuel, as the main raw material for making new glucose, and as a signaling molecule. Blood lactate is the balance between that production and that clearance.

Plot blood lactate against speed or power during a step test and you get a curve that stays nearly flat, rises gently, then bends sharply upward. Faude's review describes the two breakpoints on that curve as the aerobic-anaerobic transition.

LT1, the first threshold. This is the intensity where lactate first rises above its baseline. Below it you can work for hours, the effort feels conversational, and the metabolic cost of each extra minute is low. LT1 marks the top of the moderate intensity domain and the top of what most coaches call Zone 2.

LT2, the second threshold. This is the highest intensity where lactate production and clearance still balance, so blood lactate settles at a raised but stable level. Push a little harder and lactate climbs for as long as you keep going. LT2 marks the top of the heavy domain. Above it you are in the severe domain, where fatigue arrives in minutes.

Jamnick and colleagues reviewed the evidence for each marker in 2020. They found reasonable support for LT1 and the first ventilatory threshold as the line between the moderate and heavy domains. For the line between heavy and severe, they found good support only for critical power and critical speed.

02How the threshold gets defined

The 25 concepts in Faude's review fall into three groups. Some use a fixed lactate concentration. Some detect the first rise above baseline. The rest try to find the maximal lactate steady state or a sharp change in the slope of the curve. The main ones are below.

Fixed concentrations. The 4 mmol/L marker comes from German and Scandinavian work of the late 1970s and early 1980s. Kindermann, Simon and Keul called it the aerobic-anaerobic threshold in 1979, Sjödin and Jacobs named it OBLA in 1981, and Heck and colleagues published a justification of it in 1985. It is simple to read off a curve, and it correlates well with race times. Its problem is that your own steady-state lactate may sit far from 4. Beneke measured the maximal lactate steady state in 26 young men in 2003 and found an average of 5.1 mmol/L with a standard deviation of 1.3, so roughly two in three sat between about 3.8 and 6.4 mmol/L. For one person 4 mmol/L lands above LT2, and for another well below it.

Fixed concentrations also move with diet. Maassen and Busse showed in 1989 that 2 mmol/L fell at about 60 percent and 4 mmol/L at about 75 percent of maximal workload in trained and untrained people after a high-carbohydrate diet and rest. When trained cyclists trained on a low-carbohydrate diet, both markers shifted to higher workloads. Their thresholds looked better, yet they lasted far longer at 65 percent of max workload after the high-carbohydrate diet and rest. A lactate curve that shifts right after a hard block can mean fitness, or it can mean empty glycogen stores.

Dmax. Cheng and colleagues proposed Dmax in 1992 to remove the judgment call from reading a curve. The method draws a line from the first to the last point and picks the point on the curve farthest from it. Pallarés and colleagues tested 14 trained cyclists twice in 2016. Most lactate threshold methods repeated with a coefficient of variation of 3.4 to 3.7 percent. Dmax repeated at 10.3 percent. One likely reason is that Dmax depends on the final data point, which moves with how hard you push the last stage. In the same study, the first lactate rise matched the first ventilatory threshold, and the workload where lactate sat 2 mmol/L above that point matched the second.

Maximal lactate steady state. MLSS is the reference most lactate methods are validated against. You ride or run for 30 minutes at a constant workload on several different days. The highest workload at which lactate stays flat after the first 10 minutes is your MLSS. Beneke's 2003 comparison supports a rise of no more than 1.0 mmol/L from minute 10 to minute 30 as the criterion, since shorter tests gave different answers. The cost is time. Studies such as Keir's and Inglis's needed two or three separate 30-minute trials per person after an initial step test.

Test design changes the answer. Jamnick and colleagues put 17 trained cyclists through step tests with stages of 1, 3, 4, 7 and 10 minutes in 2018 and calculated 14 different thresholds from four of those tests. Many traditional methods failed to match MLSS, and a modified Dmax from 4-minute stages came closest, within 1.1 W on average. A threshold number means little without the protocol and method that produced it.

03Anaerobic threshold, ventilatory thresholds and critical power

These terms describe related events. They are not interchangeable, and treating them as one number is a common source of mis-set training zones.

Anaerobic threshold. Karlman Wasserman's original idea tied the rise in blood lactate to a shortage of oxygen in working muscle. Poole, Rossiter, Brooks and Gladden reviewed 50 years of that debate in 2021. They concluded that oxygen shortage is not the main cause of rising lactate. Lactate accumulates because glycolysis speeds up with work rate. The name survives in coaching, usually as a label for LT2. The anaerobic threshold entry covers that usage in more detail.

Ventilatory thresholds. A lab test with gas analysis finds two breathing breakpoints. At VT1, carbon dioxide output and ventilation start rising faster than oxygen uptake. At VT2, also called the respiratory compensation point, ventilation rises faster than carbon dioxide output as your body starts compensating for falling blood pH. VT1 lines up with LT1, and below it you can speak in full sentences. VT2 lines up with LT2, and above it you can manage only a few words at a time. Pallarés found most ventilatory markers slightly more repeatable than the lactate methods, with coefficients of variation of 1.6 to 3.5 percent.

Critical power. Critical power, or critical speed for runners, is the asymptote of your power-duration curve, estimated from several all-out efforts of different lengths. Jones, Burnley, Black, Poole and Vanhatalo argued in 2019 that critical power is the true boundary between exercise you can hold in a steady state and exercise you cannot. They showed that MLSS, as usually measured, sits somewhat lower, and that effort at critical power lasts only about 20 to 30 minutes.

The research is split on how close these markers sit. Keir and colleagues tested 12 men in 2015 and found critical power, MLSS and the respiratory compensation point at the same oxygen uptake, about 3.3 L/min. In power terms, critical power (226 W) and MLSS (223 W) matched, while the respiratory compensation point from a ramp test fell at 262 W. Galán-Rioja and colleagues pooled nine studies in 2020 and found MLSS underestimated critical power by 11 percent and VT1 by 30 percent, while the respiratory compensation point overestimated it by 6 percent and VT2 by 21 percent.

In practice, LT2, MLSS, critical power, VT2 and functional threshold power all describe the border between hard work you can sustain and work you can't. Gaps of 6 to 30 percent in pooled data are big enough to put a session in the wrong domain, so pick one marker and stay with it.

04Typical values

Thresholds vary so much between people that population ranges do little to place yours. Iannetta and colleagues tested 100 adults, 46 women and 54 men, in 2020. LT1 fell anywhere from 45 to 74 percent of VO2 max and from 60 to 90 percent of max heart rate. MLSS, the best lab estimate of LT2, fell between 69 and 96 percent of VO2 max and between 75 and 97 percent of max heart rate. Women reached both thresholds at a higher percentage of their maximums than men.

Training moves both thresholds toward a larger share of VO2 max. A common summary places LT1 near 50 to 60 percent of VO2 max and LT2 near 60 to 70 percent in untrained people, and LT1 near 70 to 75 percent and LT2 near 80 to 90 percent in well-trained athletes. Endurance training shifts the whole curve to the right, and two athletes with the same VO2 max can have very different curves.

That spread is why percentages of max heart rate or VO2 max can't substitute for a threshold test. The same 85 percent of max heart rate puts one person below LT2 and another well above it. Iannetta's team concluded that fixed-percentage prescription controls the metabolic stimulus poorly.

05Field tests and how far off they run

A lab step test with blood samples, or a gas analysis test, gives the most direct answer. Several field tests try to get close without a lab, each with a known error.

The 30-minute time trial. McGehee, Tanner and Houmard compared four field tests against a lab threshold at 4 mmol/L in 27 competitive runners and triathletes, and the 30-minute solo time trial did best. Its standard error of 8 bpm means roughly one runner in three landed more than 8 bpm from their lab threshold heart rate.

The 20-minute cycling test. Functional threshold power is conventionally 95 percent of a 20-minute all-out effort. Borszcz, Tramontin and Costa found that estimate matched MLSS on average in 15 trained cyclists, with 95 percent limits of agreement of plus or minus 9.2 percent. For a rider with an FTP of 250 W, the true MLSS could plausibly sit anywhere from about 230 to 270 W. Inglis and colleagues found MLSS at 88.5 percent of 20-minute power in 18 well-trained people. In the 10 who returned after seven months of training, MLSS rose by 12 W on average while 20-minute power didn't change. MacInnis, Thomas and Phillips tested trained cyclists in 2019 and found 20-minute power near 343 W against 60-minute power of 309 W, a ratio closer to 90 percent than 95. The 20-minute test was highly reliable and strongly related to hour power, so the test itself holds up and the conversion factor is the uncertain part. A rider who averages 265 W for 20 minutes gets an FTP of 252 W at the 95 percent convention, or about 239 W at the ratio MacInnis measured. Pace the effort evenly, since starting too hard and fading is the usual error.

The Conconi test. Conconi's 1982 test looks for the point where heart rate stops rising in a straight line with running speed. Jones and Doust retested it in 14 trained runners in 1997. Only nine showed a deflection at all. Where it appeared, it came at 5.08 m/s against a lactate turnpoint of 4.39 m/s, and at 186 bpm against 172 bpm. Runners held a pace just below the lactate turnpoint for the full 30 minutes, and a pace just below the deflection speed for 15.9 minutes on average.

A lab result carries its own error. Bonaventura and colleagues compared six handheld lactate analyzers in 2015. The portable units repeated within about 0.5 mmol/L across the 1 to 10 mmol/L range and tended to read low at high concentrations. At LT1, where the rise you are looking for may be under 1 mmol/L, that noise matters.

06What training moves it

Endurance training raises your threshold mainly by building mitochondria. Holloszy and Coyle described the chain in 1984. More mitochondrial capacity means a smaller disturbance at any given workload, slower use of glycogen, more fat oxidation and less lactate production at the same pace.

How much you gain depends on where you start. Londeree pooled 85 training groups from 34 studies in 1997. Sedentary people improved oxygen uptake at threshold by a large effect size of 2.32, and training near threshold was enough to produce it. Already-trained people gained less, with an effect size of 0.63 that did not reach significance, and tended to respond better to high-intensity training than to lower intensities (effect sizes of 1.1 against 0.4). Detraining lowered thresholds.

More threshold work is not the obvious answer for trained athletes. Stöggl and Sperlich randomized 48 well-trained endurance athletes to four nine-week programs in 2014. Speed or power at 4 mmol/L rose 8.1 percent with polarized training and 5.6 percent with high-intensity intervals. The group that trained mostly at threshold saw no significant change. Beneke's 2011 review of lactate testing put it bluntly, stating there is no evidence that training at threshold intensity is especially effective. It also noted that high-volume endurance athletes spend 70 to 90 percent of their training below the first threshold.

The lactate shuttle explains why mixed training works. The monocarboxylate transporters that move lactate between cells respond to training. MCT1, found mainly in oxidative fibers and the heart, increases with endurance training, and MCT4 helps glycolytic fibers export lactate (Brooks, 2018). From this view, the threshold is the point where lactate production outruns the shuttle. Threshold intervals train lactate transport at high rates of production, and low-intensity volume builds the oxidative machinery that consumes lactate. Both raise LT2 by different routes.

A reasonable plan for most people is a large base of easy work below LT1, a smaller share of intervals above LT2, and threshold sessions as one ingredient among several. Retest after each training block under the same conditions. Test after a rest day with normal carbohydrate intake, so glycogen depletion doesn't pass for fitness.

07What your watch's heart rate can and can't tell you

A watch measures heart rate, and heart rate at threshold differs so much between people that no formula can place it from max heart rate alone. Iannetta's range of 75 to 97 percent of max heart rate for MLSS is the whole problem in one number.

Heart rate accuracy. Optical sensors are good enough for steady efforts. Gillinov and colleagues tested five wrist and arm monitors on 50 adults in 2017. Across all exercise types the Apple Watch agreed with an ECG at a concordance of 0.92, behind a Polar chest strap at 0.996. Accuracy dropped with activity type, and none of the devices was accurate on an elliptical trainer with moving arm levers. For threshold intervals, where a few bpm matter, a chest strap is the safer choice.

Zones from max heart rate. Apple describes its Heart Rate Zones as percentages of your maximum heart rate, personalized from your health data and calculated only when your date of birth is in the Health app. You can set the zone limits manually in the Workout settings. Zones built this way are useful for tracking effort, and they carry no information about where your thresholds sit.

Watch estimates of threshold. Garmin offers a lactate threshold feature on some running watches. Its manuals describe the estimate as built from your user profile and VO2 max estimate, with automatic detection during steady high-intensity runs and a guided test, and the feature requires a Garmin chest heart rate monitor. Treat any watch estimate as a starting point to check against a time trial or lab test.

Heart rate variability during exercise. A newer approach reads the beat-to-beat pattern during a ramp test. The measure is DFA alpha 1, a fractal index of heart rate variability that falls as intensity rises. Rogers and colleagues found that DFA alpha 1 reaching 0.75 matched VT1 in 15 participants on a treadmill ramp, at an average of 154 bpm against 152 bpm for gas-exchange VT1. A value of 0.5 matched VT2 less tightly, with average heart rates of 171 and 174 bpm and a correlation of 0.78. The signal needs clean beat-to-beat intervals throughout the effort. Even a Polar H10 chest strap showed limits of agreement of about 58 to minus 41 percent for DFA alpha 1 against ECG at high intensity in Schaffarczyk's 2022 validation. The method looks promising for LT1 with a chest strap and remains unproven for LT2.

What watch data does well. Once you know your threshold heart rate from a test, the watch can keep you honest during sessions, and it can show whether heart rate at a fixed pace is drifting down over months as fitness improves. It can also show drift within a session. Kindermann's skiers ran for 30 minutes at their 4 mmol/L speed, and their heart rate rose slowly but steadily the whole time, averaging above 170 bpm. Holding a fixed heart rate cap for a long threshold effort means slowing down as it goes, and holding a fixed pace means watching heart rate climb.

08How Titan uses lactate threshold

Titan does not measure blood lactate or estimate a lactate threshold. It sorts the heart rate recorded during your workouts into five heart rate zones, each starting at a percentage of your max heart rate. By default the zones start at 53, 74, 81, 87 and 93 percent. Titan sets your max as the higher of the highest heart rate in Apple Health over the last 30 days and 220 minus your age, unless you enter a custom value. These defaults are percentages of max, so like Apple's zones they are not tied to your thresholds.

If you have a threshold result, you can tie Titan's zones to it. Tap your avatar on Today to open You, open Your Training, and under Heart Rate tap Heart Rate Zones. Each zone's start moves in 1 percent steps.

Here is an example. A runner with a max heart rate of 190 bpm finds LT1 at 148 bpm from a talk test and LT2 at 171 bpm from a 30-minute time trial. 148 is 78 percent of 190, so setting Zone 3 to start at 79 percent makes Zone 2 end at 148 bpm. 171 is 90 percent of 190, so setting Zone 4 to start at 90 percent puts the start of Zone 4 at threshold. Easy runs then belong in Zones 1 and 2, threshold work sits at the bottom of Zone 4, and intervals land in Zone 5.

Changing zones changes your numbers elsewhere in Titan. Training load and your Exertion score weight each workout minute by its zone number, one point per minute in Zone 1 up to five in Zone 5. Moving the boundaries moves minutes between zones, so expect a step change in load after you edit them. Titan reads HRV overnight for Recovery and does not compute DFA alpha 1 during workouts.

The performance physiology guide connects thresholds to oxygen delivery, fuel use and VO2 max.

09References

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