Lactate Testing for Athletes: Beyond Blood Lactate Threshold
Table of Contents
Introduction
A sensation familiar to almost everyone is the buildup of “lactic acid” in the muscles. In reality, lactate is produced continuously, even at rest, but begins to accumulate as exercise intensity increases and production exceeds the body’s ability to clear it. This point of rising lactate concentration is commonly used as a marker of aerobic fitness.
Lactate testing for athletes has evolved far beyond simply identifying “the threshold” at which effort becomes unsustainable. Modern lactate assessment commonly identifies two practically useful metabolic breakpoints, reveals how training is reshaping the aerobic system, and provides a physiological anchor for structuring training zones with a precision that heart rate alone cannot deliver.
Research in trained endurance athletes consistently shows that lactate threshold often correlates more strongly with race performance than VO₂max alone [1] — yet many endurance athletes still train by feel, by heart rate, or by generic percentage-based zones calibrated from estimates rather than measured physiology. I wrote this article to explain what lactate testing actually measures, why two thresholds are more useful than one, and how to translate the data into evidence-based training. For an overview of which physiological markers are worth testing in the first place, see Which Blood Tests Do Athletes Actually Need?
The Lactate Paradigm Shift: Fuel, Not Waste
The foundational misconception about lactate needs addressing before discussing testing. For most of the 20th century, lactate was classified as a metabolic waste product of anaerobic glycolysis — an unwanted byproduct to be cleared as quickly as possible [2]. Current evidence has substantially revised this view.
Lactate is now understood as a multifunctional metabolite: a readily accessible fuel shuttled throughout the body, a buffer bridging glycolysis and oxidative phosphorylation, and an active signaling molecule with roles in immune regulation, anti-inflammation, wound healing, and metabolic coordination [2]. The lactate shuttle — the continuous transfer of lactate between producing and consuming cells — operates even at rest and across the full intensity spectrum of exercise [3].
Importantly, lactate is not simply a byproduct — it is also a usable fuel. Once produced, lactate can be transported and converted back to pyruvate, where it is oxidized in mitochondria to produce energy. This reframing matters practically. Elevated blood lactate during a test does not indicate metabolic failure. It reflects a dynamic balance between production and clearance — and it is precisely the shape of that balance curve across intensities that makes lactate testing clinically and athletically informative [3].
I often think of it this way: when oxidative metabolism cannot keep up, lactate oxidation does not proceed “all the way.” In that sense, lactate still contains significant potential for further oxidation once mitochondrial capacity is restored. This is why lactate is not simply a waste product, but a temporary energy carrier that can be reused when conditions allow.
While lactate plays an essential physiological role, its interpretation changes significantly in clinical settings.
Lactate in Clinical Practice: Marker of Metabolic Stress
In clinical practice, I routinely use lactate as a tool in the assessment of critically ill patients. It is typically obtained as part of a blood gas analysis (arterial or venous), and is often among the first-line investigations in acutely unwell patients. In conditions such as intoxications, respiratory failure, and infections like sepsis, elevated lactate levels can reflect impaired tissue perfusion, metabolic stress, or disrupted cellular respiration.
I interpret lactate as a marker of metabolic stress and physiological strain, reflecting the balance between lactate production and clearance under increased physiological demand. In its most severe form, lactic acidosis represents a medical emergency that typically requires hospital management, often in an intensive care setting. It is also important to recognize that certain medications, such as metformin, can increase the risk of lactic acidosis in vulnerable patients, which is why I often temporarily withhold it during acute illness or infection.
Lactate, Performance, and Metabolic Demand
In both clinical and athletic settings, rising lactate reflects a mismatch between its production and clearance — often occurring when glycolytic flux increases relative to the capacity for oxidative metabolism. Glycolytic flux converts glucose into pyruvate, which serves as a key substrate for mitochondrial energy production.
Contrary to common belief, oxygen availability is often not the primary limitation. VO₂max reflects the maximal capacity to deliver and utilize oxygen, but it is not always the main determinant of endurance performance. In less trained individuals, VO₂max often represents a key limiting factor. However, in well-trained individuals, performance is more often constrained by mitochondrial capacity — the ability to utilize oxygen efficiently at the cellular level.
In both education and public discussion — at least in my experience in Finland — VO₂max is often presented as the gold standard of fitness. While this is a useful simplification, particularly in less trained individuals where oxygen delivery can indeed be limiting, it becomes less accurate at higher levels of performance, where mitochondrial capacity and metabolic efficiency play a more decisive role.
As energy demand increases, glycolytic flux can outpace the rate at which pyruvate is oxidized in mitochondria, leading to increased lactate formation. Importantly, this rise in lactate does not necessarily indicate a lack of oxygen — especially in critical illness, where hyperlactatemia often occurs despite adequate oxygen availability. A key distinction, however, is that in exercise, mitochondrial function is typically preserved and well-adapted, whereas in critical illness it may be impaired, contributing to lactate accumulation even when oxygen delivery is sufficient.
In simple terms, fitter individuals have greater mitochondrial capacity (along with higher VO₂max) allowing more pyruvate to be oxidized before it is converted to lactate and delaying the point at which lactate begins to accumulate. This reflects not fundamentally different mitochondria, but a greater number of them and a higher overall capacity for aerobic energy production.
This difference is often observable in practice and reflects a qualitative difference in subjective experience between central and peripheral limitations. In less trained individuals, exercise is often limited by cardiorespiratory strain, experienced as shortness of breath. In contrast, well-trained individuals more often reach a point where fatigue is driven by peripheral metabolic stress, even while breathing remains manageable. I have also noticed this myself as an interval-based athlete in combat sports, where the limiting sensation is often not breathlessness, but a more diffuse, systemic sense of fatigue driven by metabolic stress.
Lactate Testing for Athletes: What It Actually Measures
In lactate testing for athletes, the standard understanding has traditionally focused on a single “threshold” — the intensity at which blood lactate begins accumulating rapidly. The physiological picture is more nuanced and more useful: two operational breakpoints are commonly identified during incremental exercise, each with different training implications [4].
Athletes often differ in their lactate profiles and the way they operate relative to their thresholds. For this reason, it is also clinically important to clarify which lactate threshold is being referred to, as well as the type of sport the patient engages in.
LT1 — The Aerobic Threshold
LT1 marks the lowest exercise intensity at which blood lactate begins to rise measurably above resting values. Below LT1, the body operates in predominantly aerobic, fat-dominant metabolism; above it, carbohydrate contribution begins to increase while lactate clearance still keeps pace [4]. An athlete can generally sustain efforts below LT1 for extended durations with stable lactate concentrations. LT1 defines the upper boundary of Zone 1 in a common three-zone model, and its position on the intensity curve is directly trainable — a rightward shift indicates the athlete can sustain higher velocities or power outputs within predominantly aerobic metabolism.
Endurance athletes typically spend most of their time below the second lactate threshold, where lactate production and clearance remain balanced or clearance slightly exceeds production. As a result, they are able to sustain prolonged efforts without significant accumulation — in their case, LT1 (the aerobic threshold) becomes particularly relevant for structuring training.
LT2 — The Anaerobic Threshold / Maximal Lactate Steady State
LT2 represents the highest intensity at which lactate production and clearance remain in equilibrium — the maximal lactate steady state (MLSS). Above LT2, blood lactate accumulates continuously and the effort becomes unsustainable over extended durations [4]. The relationship between marathon running pace and MLSS is well established, with average marathon velocity in trained athletes closely approximating MLSS workload [5]. LT2 therefore serves as a primary anchor for threshold training, race-pace work, and time-trial intensity prescription.
While LT2 defines the highest sustainable intensity, performance above this point depends on what is often referred to as “lactate tolerance” — the ability to sustain effort despite continuous lactate accumulation. In my experience, this is particularly relevant in combat sports, where athletes repeatedly operate above LT2 during high-intensity efforts.
Interval-based athletes — such as combat sports athletes, including myself — spend a significant portion of their performance time above this equilibrium point. In these conditions, lactate accumulates as production exceeds clearance, and the ability to tolerate and perform under this metabolic stress becomes critical. For these athletes, both LT2 and lactate tolerance are key performance determinants.
Why Lactate Adds Information Beyond VO₂max and Heart Rate
VO₂max is widely regarded as a key marker of aerobic capacity, but it is not always the strongest predictor of endurance performance — lactate threshold often correlates more strongly with race outcomes [1]. Kumagai et al. found correlation coefficients between anaerobic threshold and running performance of r = 0.95 for 5,000 m and r = 0.84 for 10,000 m in young trained runners, compared with r = 0.65 and r = 0.67 respectively for VO₂max in the same sample [6]. Exact values vary by study population and methodology, but the general pattern that lactate threshold accounts for more performance variance than VO₂max alone is consistently replicated [1].
As discussed earlier, VO₂max reflects the maximal capacity to take in, transport, and utilize oxygen, but it does not fully capture how efficiently that oxygen is used at submaximal intensities. In many cases — particularly in well-trained individuals — oxygen delivery itself is not the primary limiting factor; rather, performance is constrained by mitochondrial capacity and metabolic efficiency at the cellular level.
Heart rate zones, while practical and accessible, have well-documented limitations as standalone tools. Heart rate drifts with heat, dehydration, caffeine, fatigue, and emotional stress. Two athletes exercising at the same heart rate can be operating in meaningfully different metabolic states. Direct blood lactate measurements add metabolic information that heart rate alone cannot provide [1]. For a deeper look at how resting heart rate changes signal overtraining and why heart rate monitoring has limits even at rest, see the dedicated guide on drantti.com.
For time-trial prediction specifically, direct blood lactate measurements have been shown to correlate more strongly with 1-hour time-trial performance than indirect methods using pulmonary gas exchange, in both cool and hot conditions [7]. This relationship, however, depends on training status, and is particularly evident in well-trained individuals, where performance is more closely linked to metabolic efficiency than to maximal oxygen delivery alone — a finding with direct implications for race preparation and pacing strategy.
For this reason, I recommend selecting the testing method based on the individual. In more sedentary or less trained patients, VO₂max testing is often the more appropriate first-line assessment, as oxygen delivery may represent a primary limitation. In contrast, in well-trained or elite athletes, lactate testing is often more informative, as performance is more closely linked to metabolic efficiency and threshold dynamics.
Summary
Lactate is not a waste product, but a central component of human metabolism — a dynamic fuel, signaling molecule, and marker of physiological stress. Lactate testing provides insight not only into performance limits, but into how the body produces, utilizes, and regulates energy under load. The distinction between LT1 and LT2 allows for a more precise understanding of training intensity, separating sustainable aerobic work from maximal steady-state effort and, beyond that, from lactate tolerance-driven performance.
While VO₂max reflects the maximal capacity to deliver and utilize oxygen, it does not fully capture how efficiently that capacity is used — particularly in well-trained individuals, where mitochondrial function and metabolic efficiency become the primary constraints. Lactate testing therefore offers a more practical and individualized framework for training prescription, performance monitoring, and pacing strategy.
In clinical practice, lactate serves a different but equally important role, acting as a marker of metabolic stress and physiological imbalance. Understanding lactate across both contexts — performance and pathology — highlights a unifying principle: rising lactate reflects not failure, but a shift in the balance between energy demand and metabolic capacity.
For athletes, coaches, and clinicians alike, the value of lactate lies not in a single number, but in the shape of the response — and in understanding what that response reveals about the underlying physiology.
References
- https://pmc.ncbi.nlm.nih.gov/articles/PMC2769631/
- https://pubmed.ncbi.nlm.nih.gov/33596629/
- https://pmc.ncbi.nlm.nih.gov/articles/PMC12619971/
- https://pubmed.ncbi.nlm.nih.gov/19453206/
- https://pubmed.ncbi.nlm.nih.gov/12744715/
- https://pubmed.ncbi.nlm.nih.gov/7201924/
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3137529/
