Lactate, Lactic Acid, and Lactate-Threshold Assessment: A Structured Narrative Review for Endurance and Hybrid-Sport Practice
Abstract
Background: Lactate is one of the most widely used but frequently misunderstood markers in exercise physiology. In applied sport settings, "lactic acid" is still often blamed for fatigue, soreness, and the burning sensation of hard exercise, even though lactate predominates at physiological pH and functions as an important metabolic intermediate rather than a simple waste product. Objective: This structured narrative review clarifies the distinction between lactate and lactic acid, synthesizes lactate-threshold terminology, and translates threshold-informed evidence into practical guidance for endurance and hybrid athletes. Methods: A targeted literature synthesis was conducted using peer-reviewed mechanistic, methodological, and applied papers on lactate metabolism, blood lactate testing, threshold concepts, maximal metabolic steady state, exercise-intensity prescription, interval training, and wearable lactate monitoring. Because no original participant dataset is reported, the manuscript is positioned as a structured narrative review rather than an empirical study. Synthesis: The evidence supports lactate as a central metabolite, transport substrate, and interpretive marker of exercise intensity. However, "lactate threshold" is not a single universal event: LT1, LT2, OBLA, MLSS/MMSS, ventilatory thresholds, and critical power or speed each represent different operational constructs. Fixed percentages of HRmax, VO2max, or maximal speed often fail to normalize internal physiological strain across individuals, whereas threshold-based anchors can improve individualization when the testing method and target training domain are explicit. Field tests and wearable technologies can support monitoring, but they should be treated as estimates rather than replacements for validated protocols. Conclusion: Lactate-informed coaching is strongest when the practitioner reports the threshold definition, understands the limits of the measurement method, and interprets results alongside pace, power, heart rate, rating of perceived exertion, performance durability, and recovery trends.
Keywords
Lactate; lactic acid; lactate threshold; LT1; LT2; OBLA; maximal lactate steady state; maximal metabolic steady state; exercise-intensity prescription; endurance training; hybrid athletes; wearable biosensors
Highlights
- Lactate should be framed as a metabolically useful molecule and interpretive marker, not as the direct toxic cause of fatigue.
- Threshold terminology must be explicit because LT1, LT2, OBLA, MLSS/MMSS, ventilatory thresholds, and critical power or speed are not interchangeable.
- Threshold-based prescription can better individualize internal load than fixed percentages of HRmax, VO2max, or maximal pace, but the best anchor depends on the session goal.
- Field tests, smartwatches, and wearable lactate sensors can assist coaching decisions, but current evidence supports cautious use rather than uncritical substitution for laboratory testing.
Abbreviations
BLa, blood lactate concentration; CP, critical power; CS, critical speed; HR, heart rate; HRmax, maximal heart rate; LT, lactate threshold; LT1, first lactate threshold; LT2, second lactate threshold; MLSS, maximal lactate steady state; MMSS, maximal metabolic steady state; OBLA, onset of blood lactate accumulation; RPE, rating of perceived exertion; VO2max, maximal oxygen uptake.
Introduction
Lactate sits at the intersection of metabolism, performance testing, and coaching language. It is measured in laboratories, discussed on training floors, shown on wearable-technology roadmaps, and used to organize training zones for endurance athletes. Yet it is also one of the most oversimplified concepts in sport science. Public-facing explanations still use "lactic acid" as a shorthand for pain, fatigue, or post-exercise soreness, while some modern discussions swing too far in the opposite direction and imply that lactate is always beneficial or simple to interpret. A publication-ready account needs to avoid both errors. Lactate is useful, but its usefulness depends on method, context, and interpretation.
At physiological pH, lactic acid is largely dissociated into lactate and hydrogen ions. This makes the routine statement that "lactic acid builds up in the muscles" chemically imprecise. More importantly, blood lactate concentration is not a direct one-to-one measure of acid, fatigue, anaerobic metabolism, or exercise quality. It reflects the dynamic balance among lactate appearance, clearance, transport, oxidation, gluconeogenic use, and distribution across tissues (Gladden, 2004; Goodwin et al., 2007). Lactate is therefore better understood as an active metabolite and a marker of metabolic flux than as an inert waste product.