Anaerobic threshold training is structured exercise performed at or near the intensity where lactate begins accumulating faster than your body can clear it. This intensity sits in a physiological sweet spot: hard enough to force meaningful adaptations in how your muscles use fuel and shuttle lactate, but sustainable enough that you can maintain it for extended periods without collapsing into exhaustion. For endurance athletes and fitness-minded people alike, it represents one of the most reliable ways to improve the pace or power you can hold over long efforts. The concept sounds straightforward, but the science behind it has been debated for over fifty years, and the way people talk about “the threshold” often muddies more than it clarifies.
What Happens at the Threshold
For decades, the standard explanation was that your muscles run low on oxygen at a certain exercise intensity, forcing them to rely on anaerobic energy pathways that produce lactate as a byproduct. That story has largely been overturned. Researchers now recognize that insufficient oxygen is not the primary driver of lactate accumulation. Instead, lactate rises because your muscles ramp up their use of carbohydrate-based fuel as exercise gets harder, and the rate of lactate production simply outpaces your body’s ability to clear and recycle it.1PubMed. The anaerobic threshold: 50+ years of controversy Lactate itself is not a waste product. It is a crucial energy source, a building block for making new glucose, and a signaling molecule that triggers training adaptations.
Several processes converge to create the threshold. Fat oxidation reaches its maximum capacity around this intensity, so any further increase in effort has to come from carbohydrate metabolism, which generates lactate. Stress hormones called catecholamines also appear to be essential: without them, the threshold phenomenon does not seem to occur at all.2PubMed. Possible mechanisms of the anaerobic threshold. A review Other factors like motor unit recruitment patterns, muscle temperature, and lactate transport rates all play supporting roles.
There is also an important intensity concept above the threshold called critical power (or critical speed, for runners). Critical power marks the boundary between exercise you can sustain for a long time, even if it is hard, and exercise that will inevitably force you to stop. This boundary occurs at a metabolic rate often well above the lactate threshold.3PubMed. The anaerobic threshold: 50+ years of controversy Training between your lactate threshold and your critical power is the zone most people mean when they talk about threshold training.
Why the Terminology Is So Confusing
If you have ever tried to read about threshold training and ended up more confused than when you started, you are not alone. The field uses a dizzying number of terms that sound interchangeable but are not. Lactate threshold, ventilatory threshold, onset of blood lactate accumulation, maximal lactate steady state, anaerobic threshold, respiratory compensation point: these all describe slightly different measurements taken in slightly different ways. Coaches and athletes often use them as synonyms, but research shows they correspond to meaningfully different exercise intensities.
A meta-analysis comparing these thresholds against critical power found that while most correlated strongly with critical power, they consistently under- or overestimated it. The first ventilatory threshold underestimated critical power by about 30%, maximal lactate steady state underestimated it by about 11%, and the respiratory compensation point overestimated it by roughly 6%. The authors concluded that none of the common thresholds should be considered synonymous with critical power or with each other.4PubMed. Relative Proximity of Critical Power and Metabolic/Ventilatory Thresholds: Systematic Review and Meta-Analysis
For practical purposes, this means you should treat any single threshold number as an approximation, not a hard boundary. If a coach tells you to train “at threshold,” what they usually mean is the moderate-to-hard zone where conversation becomes difficult but you are not gasping. That general intensity zone is what matters for training, even if the precise physiological label is debatable.
How Your Body Adapts to Threshold Training
Training near your threshold triggers a cascade of changes that make your muscles better at producing and clearing lactate simultaneously. One of the most well-documented adaptations involves lactate transporter proteins embedded in muscle cell membranes. After nine weeks of endurance training at about 75% of peak oxygen uptake, researchers found that MCT1, a transporter that shuttles lactate into mitochondria and between cells, increased by 60 to 90% in different muscle compartments. MCT4, another transporter on the cell surface, also increased, though the response was more variable between individuals.5PubMed. Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle These changes mean trained muscles can move lactate around more efficiently, using it as fuel rather than letting it pile up.
Mitochondrial enzyme activity also increases with threshold-oriented training, reflecting greater capacity to burn fuel aerobically. Citrate synthase activity, a standard marker of mitochondrial density, rose by 75% in that same nine-week study.6PubMed. Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle There were also shifts in muscle fiber composition, with slow-twitch fibers becoming a larger proportion of the muscle. Since slow-twitch fibers are inherently better at aerobic metabolism, this shift alone helps push the threshold to a higher workload.
Buffering capacity is another piece of the puzzle. Inside your muscle fibers, a molecule called carnosine helps soak up the hydrogen ions that accumulate during hard exercise and cause that burning sensation. Carnosine contributes roughly 20% of buffering in slow-twitch fibers and up to 46% in fast-twitch fibers.7PubMed Central. Effects of Beta-Alanine on Muscle Carnosine and Exercise Performance: A Review of the Current Literature Its buffering role is especially relevant near the threshold, where hydrogen ion accumulation starts to accelerate. Higher baseline carnosine levels have been positively correlated with rowing performance, and increases in muscle carnosine predicted performance improvements in competitive rowers.8PubMed. Important role of muscle carnosine in rowing performance
Workout Formats for Threshold Training
There are two broad approaches to training at threshold intensity: sustained continuous efforts and structured intervals. Both work. A study comparing a continuous method performed at the upper boundary of the heavy-intensity domain (around 80% of peak treadmill speed) against a high-intensity interval protocol found that after a short training block, both groups improved their VO2max, critical velocity, and velocity at lactate threshold by comparable amounts, with no significant difference between them.9PubMed. Aerobic adaptations following two iso-effort training programs: an intense continuous and a high-intensity interval
So if both work equally, why bother with intervals? The argument for lactate-guided threshold interval training is that breaking the effort into segments lets you hit higher absolute speeds or power outputs while keeping the metabolic cost in the threshold zone. The interval format also produces less central and peripheral fatigue per session, which means you can recover faster and potentially accumulate more of these key sessions per week.10PubMed Central. Does Lactate-Guided Threshold Interval Training within a High-Volume Low-Intensity Approach Represent the “Next Step” in the Evolution of Distance Running Training? For a distance runner, this could mean doing four threshold workouts a week instead of two, simply because each one leaves less residual fatigue.
A classic threshold interval session might look like 4 to 6 repetitions of 8 to 12 minutes at your threshold pace, with 2 to 3 minutes of easy jogging between them. Continuous threshold runs, often called tempo runs, typically last 20 to 40 minutes at the same intensity. Both are bread-and-butter sessions for distance runners, cyclists, and rowers.
Where Threshold Training Fits in the Bigger Picture
One of the liveliest debates in endurance coaching is how much of your training should be spent at threshold versus other intensities. The “polarized” model puts roughly 80% of training volume at low intensity and the remaining 20% at high intensity, with very little time at threshold. The “pyramidal” model emphasizes low-intensity volume but includes more threshold work. A “threshold” model, naturally, puts a larger share of training time right at the anaerobic threshold.
A scoping review of training intensity distributions found that all models effectively improved the speed or power associated with the anaerobic threshold.11The Journal of Strength & Conditioning Research. Effects of Polarized Training vs. Other Training Intensity Distribution Models on Physiological Variables and Endurance Performance in Different-Level Endurance Athletes: A Scoping Review In other words, you can improve your threshold through a wide range of training philosophies, not just by hammering threshold sessions. A study of well-trained endurance runners found that switching from a pyramidal to a polarized distribution led to the largest improvements in VO2 peak (about 3%), the speed at a fixed blood lactate concentration (about 1.5 to 1.7%), and 5-km time trial performance (about 1.5%).12PubMed Central. Effects of 16 weeks of pyramidal and polarized training intensity distributions in well-trained endurance runners
What this suggests is not that threshold sessions are unimportant but that they are most effective as part of a varied program. Spending the majority of your training time well below threshold and mixing in targeted higher-intensity sessions appears to lift the threshold as effectively as, or more effectively than, spending a large share of training at threshold itself. This is counterintuitive but has held up across multiple studies in trained athletes.
Finding Your Own Threshold
The gold standard for identifying your lactate threshold involves a graded exercise test in a lab, with blood samples taken from your finger or earlobe at each stage to measure lactate concentration. Various mathematical methods are then applied to pinpoint the intensity at which lactate begins rising sharply. The problem is that different methods yield different results. In trained cyclists, functional threshold power (a field-based 20-minute test popular in cycling) correlated strongly with the power at a fixed blood lactate concentration of 4.0 mmol/L, but the limits of agreement were wide, ranging from about 45 watts below to 51 watts above the lab value. Other lactate-derived thresholds were all significantly different from the cycling field test.13The Journal of Strength & Conditioning Research. Functional Threshold Power Is Not Equivalent to Lactate Parameters in Trained Cyclists
This means that a 20-minute field test gives you a useful training anchor, but do not treat it as a precise physiological measurement. It is a practical number for setting training zones, not a lab result.
Wearable near-infrared spectroscopy devices, which measure muscle oxygen saturation through the skin, are an emerging alternative. A validation study found that one such device could predict lactate threshold power within half of one power increment on a cycling test, but only when using proprietary software; raw muscle oxygen readings alone were not accurate enough.14PubMed Central. Validation of a novel wearable, wireless technology to estimate oxygen levels and lactate threshold power in the exercising muscle A systematic review of these portable monitors found moderate to good reliability for detecting the second ventilatory and lactate thresholds, but flagged that more research is needed on the mathematical detection methods, the effect of body fat, and the capacity to detect the first threshold.15Scientific Reports. Reliability of threshold determination using portable muscle oxygenation monitors during exercise testing: a systematic review and meta-analysis The technology is promising but not yet a plug-and-play replacement for lab testing.
How Fuel Use Shifts Around the Threshold
Your body always burns a mix of fat and carbohydrate during exercise, but the ratio shifts dramatically as intensity increases. The crossover concept describes the exercise intensity at which carbohydrate-derived fuels begin to dominate over fat as the primary energy source. Beyond this point, further increases in effort cause a progressively larger shift toward carbohydrate burning and a corresponding drop in fat oxidation.16PubMed. Balance of carbohydrate and lipid utilization during exercise: the “crossover” concept
This crossover point tracks closely with ventilatory threshold. One study found a strong correlation between the ventilatory threshold and the crossover point, and between the ventilatory threshold and the intensity of maximum fat oxidation.17Science & Sports. Relative and absolute reliability of the crossover and maximum fat oxidation points and their relationship to ventilatory threshold This relationship is why threshold training has practical implications for fueling strategy. At threshold intensity, your muscles are burning through glycogen at a high rate. If you plan to sustain that effort for an hour or more in racing, you need to take in carbohydrates during the effort. Training at this intensity also trains your gut to absorb fuel while working hard, an often-overlooked benefit.
The Respiratory Side of the Equation
Hard breathing during threshold work is not just a consequence of your muscles demanding more oxygen. The respiratory muscles themselves can become a limiting factor. When your diaphragm and intercostal muscles work hard enough, they trigger a reflex that diverts blood flow away from the legs and toward the breathing muscles, potentially reducing performance. Inspiratory muscle training has been shown to blunt this reflex. In one study, trained subjects who completed an inspiratory muscle training program nearly abolished the blood pressure response to heavy breathing and reduced their heart rate response from about 35% above baseline to 27%.18PubMed Central. Inspiratory muscle training attenuates the human respiratory muscle metaboreflex For athletes who find their breathing becomes the weak link during threshold efforts, dedicated respiratory muscle training may offer a performance edge.
Altitude and Environmental Factors
Your anaerobic threshold is not a fixed number. It shifts in response to environmental conditions, and altitude is the biggest disruptor. When researchers measured anaerobic threshold in subjects acutely exposed to altitude, they found a significant drop from about 160 watts at sea level to roughly 135 watts.19PubMed. Effect of Acute Altitude Exposure on Anaerobic Threshold Assessed by a Novel Electrocardiogram-Based Method The effect of prolonged stays at very high altitude is even more dramatic. After an extended sojourn above 5,000 meters, one study documented a drop in anaerobic threshold from about 39 to 28 mL of oxygen per kilogram per minute when tested back at sea level, along with decreases in maximal anaerobic power and total anaerobic work.20PubMed Central. Prolonged Sojourn at Very High Altitude Decreases Sea-Level Anaerobic Performance, Anaerobic Threshold, and Fat Mass
This matters for anyone training at altitude or preparing for a race at elevation. Your threshold-pace workouts need to be recalibrated, not just for the lower oxygen availability during the altitude stay, but potentially for a period after returning to lower elevations. Heat and humidity also shift the threshold downward, though the effect is less dramatic. If you rely on a specific pace or power target for threshold sessions, adjusting for environmental conditions is essential to avoid accidentally training well above threshold and accumulating unnecessary fatigue.
When Threshold Work Becomes Too Much
Because threshold training is hard enough to feel productive but sustainable enough to do frequently, it carries a specific risk: you can pile up too much of it without the acute distress signals that very hard interval sessions produce. Overreaching and overtraining are a real concern. A systematic review of hormonal markers in overtrained and overreached athletes found that resting hormone levels were mostly normal, making the condition hard to detect with standard blood tests. The clearest signal was a blunted growth hormone and ACTH response to maximal exercise, meaning the body’s stress-response system stopped reacting normally to hard efforts.21PubMed Central. Hormonal aspects of overtraining syndrome: a systematic review
Subjective signs often show up before any lab marker does. Persistently flat performance, elevated resting heart rate, disrupted sleep, loss of motivation, and that unmistakable feeling that easy runs are harder than they should be are all warning flags. The fix is straightforward but psychologically difficult: reduce volume and intensity, and give your body time to absorb the training you have already done. Elite cross-country skiers, whose dense competition schedules sometimes make a proper taper impossible, rely on repeated short recovery blocks of just a few days rather than a single long taper.22PLoS ONE. The Road to Gold: Training and Peaking Characteristics in the Year Prior to a Gold Medal Endurance Performance That approach may work for recreational athletes too, particularly those juggling training with work and family stress.
Genetics and Individual Variation
Not everyone responds to threshold training the same way, and genetics plays a role that researchers are still working to quantify. A narrative review of the genomic basis of endurance performance identified the anaerobic threshold as one of three key physiological pillars of endurance performance, alongside VO2max and movement economy. Genetic variability appears to modulate how an athlete responds to training, including aspects of metabolic efficiency, muscle fiber composition, and recovery.23PubMed Central. Genomics May Be the Key to Understanding Endurance Training Pillars Some people see their threshold rise quickly with modest training, while others grind through months of high-volume work for small gains. The variability in lactate transporter protein responses seen in lab studies hints at why: MCT4 content after training showed large differences between individuals even when they followed the same program.24PubMed. Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle
For practical purposes, this means you should evaluate your own progress rather than comparing your threshold to anyone else’s. Two runners with the same VO2max can have very different thresholds, and two runners following the same training plan can see different rates of improvement. The best training plan is the one that consistently nudges your threshold upward over months, and the only way to know if it is working is to test and track.