Lactate Threshold 2 (LT2): What Happens, Its Significance, and How to Improve It

What Happens at LT2?

At the Second Lactate Threshold (LT2), also known as the Onset of Blood Lactate Accumulation (OBLA) or Respiratory Compensation Point (RCP), the body's rate of lactate production dramatically exceeds its ability to clear lactate, leading to a rapid and sustained increase in blood lactate levels, increased reliance on anaerobic metabolism, and significant physiological stress.

Introduction to Lactate Thresholds

In exercise physiology, lactate thresholds represent critical points of intensity during progressive exercise where our body's metabolism shifts. Lactate, often mistakenly blamed solely for muscle fatigue, is a byproduct of glycolysis (the breakdown of glucose for energy) and a valuable fuel source. Our body constantly produces and clears lactate. The first lactate threshold (LT1), or aerobic threshold, marks the point where lactate begins to rise slightly above resting levels, indicating a greater reliance on carbohydrate metabolism. The second lactate threshold, LT2, is a more pronounced and critical marker of endurance performance.

Defining LT2: The Onset of Blood Lactate Accumulation (OBLA) or Respiratory Compensation Point (RCP)

LT2 signifies the intensity at which blood lactate concentrations begin to accumulate rapidly and non-linearly. It's often defined as the point where blood lactate reaches a specific concentration (e.g., 4.0 mmol/L in some models, though this can vary individually) or, more accurately, the highest exercise intensity that can be maintained while lactate production and clearance are still somewhat balanced, before an exponential rise.

Other terms for LT2 include:

• Onset of Blood Lactate Accumulation (OBLA): This term directly describes the rapid increase in blood lactate.
• Respiratory Compensation Point (RCP): From a ventilatory perspective, this is the point where ventilation increases disproportionately to oxygen consumption, as the body attempts to buffer the metabolic acidosis caused by rapidly accumulating lactate and associated hydrogen ions.

Physiological Events at LT2

When an individual crosses the LT2 threshold, a cascade of physiological events occurs as the body struggles to meet the energy demands of the escalating exercise intensity:

• Disproportionate Increase in Lactate Production: As exercise intensity rises beyond LT2, the demand for ATP (adenosine triphosphate) outpaces the capacity of the aerobic system. This forces a greater reliance on anaerobic glycolysis, which produces lactate at an accelerated rate.
• Impaired Lactate Clearance: While lactate production increases, the body's ability to transport and metabolize lactate (e.g., via the Cori cycle in the liver, or oxidation in less active muscles and the heart) becomes overwhelmed. The rate of production far exceeds the rate of clearance, leading to a rapid accumulation in the blood and muscle.
• Increased Reliance on Anaerobic Metabolism: The contribution of anaerobic energy pathways becomes significantly higher. This is unsustainable for prolonged periods due to the limited capacity of the anaerobic system and the accumulation of metabolic byproducts.
• Metabolic Acidosis and Hydrogen Ion Accumulation: The increased rate of anaerobic glycolysis also leads to a rapid production of hydrogen ions (H+). While lactate itself is not the primary cause of acidosis, its production is tightly coupled with H+ ions. The accumulation of H+ ions lowers cellular pH, disrupting enzyme function, impairing muscle contraction, and contributing directly to the sensation of fatigue.
• Respiratory Compensation: To combat the rising acidity (metabolic acidosis), the body increases its ventilation (breathing rate and depth) dramatically. This increased breathing helps to expel more carbon dioxide (CO2), which is converted from carbonic acid (H2CO3) in the blood. This process helps to buffer the H+ ions and restore pH balance, but it requires significant energy and adds to the overall physiological strain.
• Accelerated Glycogen Depletion: The heightened reliance on anaerobic glycolysis at LT2 rapidly depletes muscle and liver glycogen stores, which are the primary fuel source for high-intensity exercise.
• Increased Cardiovascular Strain: The heart rate approaches its maximum, cardiac output is extremely high, and blood pressure increases as the cardiovascular system works overtime to deliver oxygen and remove waste products. This level of exertion is very demanding on the entire system.

The Significance of LT2 for Performance

LT2 is a critical physiological marker for endurance athletes and a key determinant of performance:

• Indicator of Endurance Capacity: A higher LT2 (i.e., the ability to sustain a higher power output or speed before reaching LT2) indicates superior endurance performance. Athletes with a higher LT2 can maintain a faster pace for longer without experiencing the rapid onset of fatigue associated with significant lactate accumulation.
• Defines Critical Training Zones: LT2 often demarcates the boundary between "heavy" and "severe" intensity domains. Training at or just below LT2 (often called "threshold training" or "tempo training") is highly effective for improving endurance, as it stresses the aerobic system and enhances the body's ability to produce and clear lactate more efficiently.
• Predictor of Race Pace: For endurance events like marathons, cycling time trials, or long-distance swimming, an athlete's LT2 pace is often a strong predictor of their sustainable race pace.

Training to Improve LT2

Improving LT2 means pushing this threshold to a higher intensity, allowing you to work harder for longer before fatigue sets in. Effective strategies include:

• Tempo Runs/Threshold Training: This involves sustained efforts at or just below your LT2 pace (e.g., 20-60 minutes). The goal is to spend significant time at an intensity where lactate production is high but still manageable, forcing the body to adapt by improving lactate clearance and buffering capacity.
• High-Intensity Interval Training (HIIT): Short bursts of very high-intensity work (above LT2) followed by recovery periods. This type of training creates a strong stimulus for both aerobic and anaerobic adaptations, improving the efficiency of energy systems and enhancing the body's ability to tolerate and clear lactate.
• Long, Steady-State Efforts: While not directly targeting LT2, a strong aerobic base built through long, moderate-intensity training improves overall cardiovascular fitness and mitochondrial density, which indirectly supports a higher LT2 by enhancing fat utilization and overall aerobic capacity.

Measuring LT2

LT2 is typically measured in a controlled laboratory setting, though field-based methods can provide good estimates:

• Laboratory Testing (Lactate Profiling): An athlete performs a graded exercise test (e.g., on a treadmill or bike) where intensity is progressively increased. Blood samples are taken at regular intervals to measure lactate concentration. LT2 is identified as the point where lactate begins to rise sharply.
• Ventilatory Thresholds: During a graded exercise test with gas analysis, LT2 can be identified as the Respiratory Compensation Point (RCP), where the ventilatory equivalent for carbon dioxide (VE/VCO2) begins to rise disproportionately, indicating increased ventilation to buffer acidosis.
• Field Tests: While less precise, tests like critical power/speed tests or specific time trials can provide good estimates of an athlete's sustainable threshold power or pace, which closely correlates with LT2.

Conclusion

Understanding what happens at LT2 is fundamental for anyone serious about optimizing their endurance performance. It represents a critical physiological tipping point where the body's metabolic demands push it into a state of rapid lactate accumulation and significant physiological stress. By strategically training at and around this threshold, athletes can effectively raise their LT2, thereby enhancing their ability to sustain higher intensities for longer durations, ultimately leading to improved endurance and performance.