Lactate has a bad reputation it does not deserve. It is not what makes your legs burn, and it is not a sign that something went wrong. It is a fuel your body shuttles between cells, and its concentration in your blood is the most practical marker we have for steering training load.
What lactate actually is
Shuttle instead of wasteLactate is produced continuously, at rest as well as under load. Trained muscle does not just tolerate it, it uses it : mitochondria take lactate up and burn it. That is why a higher mitochondrial density does not only mean more fat oxidation, it also means faster lactate clearance at the same intensity. In a direct comparison of professional cyclists and less-fit men, the trained group cleared lactate at roughly 350 watts where the untrained group was already at 4 mmol/l around 150 watts.
The same difference shows up in fat metabolism. Professional endurance athletes reached maximal fat oxidation rates of about 1.1 g/min, the less-fit comparison group about 0.4 g/min, and they reached that peak at roughly 75 percent of VO2max instead of roughly 45 percent. Lactate is the marker that makes this difference visible from the outside, with a drop of blood from the earlobe or fingertip.
Zone 2 is defined physiologically as the load at which blood lactate stays stable below 2.0 mmol/l. It is the range in which the mitochondrial adaptation is driven hardest. The point is not the number on its own, it is the wattage or pace at which you personally reach it.
The two thresholds : LT1 and LT2
Where easy ends and hard beginsLT1 is the first rise above resting lactate. Below it, production and clearance are in balance, and you can hold the load for a long time. LT2 is the highest intensity at which lactate still stabilises ; above it, concentration keeps climbing and the effort has a clock on it. Between the two lies the range that feels productive and often is not : too hard to accumulate volume, too easy for a strong stimulus.
The training zones follow directly from these two points. In the intensity distribution described for elite endurance athletes, roughly 75 to 80 percent of sessions sit below the first threshold, 5 to 10 percent in the range between the thresholds, and 15 to 20 percent above the second one. Systematic reviews of highly trained distance runners describe a comparable picture with a heavier threshold share : around 80 percent low intensity, 15 percent threshold and 5 percent above it for marathon specialists, and around 65 / 25 / 10 for 5 km to 10 km specialists.
The second threshold is also where endurance performance is decided. In the classic three-factor model, race pace is the product of VO2max, the fraction of VO2max you can sustain, and movement economy. In elite male marathon runners, that sustainable fraction sits at about 85 to 90 percent of VO2max. Two athletes with the same VO2max can be minutes apart over a marathon purely because of where their second threshold lies.
Why measuring beats estimating
Formula zones have no individualZones from an age formula or from a watch estimate assume an average person. The data above show how wide the individual spread actually is : the load at which lactate reaches 4 mmol/l differed by more than a factor of two between trained and less-fit men in the same study. A percentage of an assumed maximum heart rate cannot capture that.
The second problem is specificity. Endurance adaptations are largely mode-specific : the VO2max gain shows up mainly in the modality you trained, roughly 5 to 10 percent there against roughly 1 to 3 percent on an untrained modality, and threshold shifts behave the same way. A threshold measured on the bike is a bike threshold. If you run on it, you are guessing again.
What that means per sport
Same physiology, different profileRunning distances are a useful map here, because the split between aerobic and anaerobic contribution has been measured across them in highly trained athletes. The shorter the distance, the larger the oxygen deficit ; the longer it gets, the more the aerobic side and the sustainable fraction of VO2max decide the outcome. Fitness sports can be placed on the same axis.
| Distance | Profile |
|---|---|
| 200 m | anaerobically dominant, very high oxygen deficit |
| 400 m | high oxygen deficit, moderate VO2max, moderate fractional utilisation |
| 800 m | mixed, aerobic contribution already above 50 percent |
| 1500 m | aerobically dominant, limited oxygen deficit, high utilisation threshold |
Source : Spencer & Gastin 2001, highly trained runners.
Resembles the 400 m
High oxygen deficit, moderate VO2max, moderate fractional utilisation. The consequence for training : zone 2 work belongs in the off-season, during the season strength and power keep priority. Lactate measurement mainly tells you where your easy really is, so base work does not quietly turn into another hard session.
Resembles the 1500 m
Limited oxygen deficit, high VO2max and a high fractional utilisation threshold. The consequence : a high running volume with 70 percent or more easy, strength and high-intensity work deliberately capped. Here the second threshold is the performance-limiting number, because the race is run close to it.
Peripherally maximised
World-class distance runners build the engine with volume : 160 to 220 km per week in the marathon build, 11 to 14 sessions per week, and at least 80 percent of that volume at low intensity all year round. The measured first threshold is what keeps those easy kilometres genuinely easy.
Three sports, three thresholds
Adaptations transfer between modalities, but only partly and asymmetrically : bike to run roughly 60 to 80 percent, run to bike roughly 80 to 90 percent, swimming to the land disciplines roughly 30 to 50 percent. Since threshold shifts are mode-specific too, it makes sense to determine your thresholds separately for the disciplines you actually race.
What you do with the values
Zones, control, retestFirst, the zones : your two thresholds turn into pace or wattage ranges, and every session gets a defined place. Second, the control : volume times intensity is a good predictor of both mitochondrial content and VO2max, so the plan has to be one you can absorb and recover from week after week. In the pooled human data, mitochondrial density tracks accumulated training volume closely (R² = 0.82), with the curve flattening at very high volumes.
Third, the retest. Different stimuli work on different time scales : sprint intervals raise VO2max quickly and then plateau after a few weeks, while continuous endurance work rises more slowly but for longer, and clear effects of endurance and high-intensity training accumulate over roughly five to ten intervention weeks and beyond. Trainability itself is retained across the lifespan. A retest under identical conditions is what tells you which of your two thresholds actually moved.
At StoaVita you get this in the coaching context, without diagnoses : a lactate step test in your own sport, your two thresholds, the zones that follow from them, and a defined retest date.
Measure your thresholds
Lactate diagnostics with training zones, in Tübingen and the greater Stuttgart area.
Sources (primary literature)
- San-Millán I, Brooks GA (2018). Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Med 48(2):467-479. doi:10.1007/s40279-017-0751-x
- Spencer MR, Gastin PB (2001). Energy system contribution during 200- to 1500-m running in highly trained athletes. Med Sci Sports Exerc 33(1):157-162.
- Haugen T, Sandbakk Ø, Seiler S, Tønnessen E (2022). The training characteristics of world-class distance runners. Sports Medicine - Open 8:46. doi:10.1186/s40798-022-00438-7
- Seiler S (2010). What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform 5(3):276-291. doi:10.1123/ijspp.5.3.276
- Casado A, González-Mohíno F, González-Ravé JM, Foster C (2022). Training periodization, methods, intensity distribution, and volume in highly trained and elite distance runners: a systematic review. Int J Sports Physiol Perform 17(8):1231-1243. doi:10.1123/ijspp.2021-0435
- Joyner MJ, Coyle EF (2008). Endurance exercise performance: the physiology of champions. J Physiol 586(1):35-44. doi:10.1113/jphysiol.2007.143834
- Bassett DR Jr, Howley ET (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc 32(1):70-84. doi:10.1097/00005768-200001000-00012
- Hawley JA (2002). Adaptations of skeletal muscle to prolonged, intense endurance training. Clin Exp Pharmacol Physiol 29(3):218-222. doi:10.1046/j.1440-1681.2002.03623.x
- Tanaka H (1994). Effects of cross-training. Transfer of training effects on VO2max between cycling, running and swimming. Sports Med 18(5):330-339. doi:10.2165/00007256-199418050-00005
- Granata C, Bishop DJ (2018). Training-induced changes in mitochondrial content and respiratory function in human skeletal muscle. Sports Med 48(8):1809-1828. doi:10.1007/s40279-018-0936-y
- Mølmen KS, Almquist NW, Skattebo Ø (2025). Effects of exercise training on mitochondrial and capillary growth in human skeletal muscle: a systematic review and meta-regression. Sports Med 55:115-144. doi:10.1007/s40279-024-02120-2
This article is educational and does not replace medical advice. StoaVita provides longevity and performance coaching, not medical treatment.
