The Truth About Lactate Threshold and Anaerobic Threshold: Lactate Is Not Waste but Fuel

The Rehabilitation of Lactate

Twentieth-century textbooks described lactate as a waste product of anaerobic metabolism and the culprit behind muscle soreness and fatigue. George Brooks’ lactate shuttle theory completely overturned this view: lactate is a normal product of glycolysis, produced continuously even with ample oxygen supply, and it is taken up and reused as fuel by the heart, slow-twitch muscle fibers, the liver, and the brain. The fatigue and burning sensation are caused by the accompanying hydrogen ions (H+) and phosphate, not by lactate itself.

Two Thresholds, Two Physiological Events

Blood lactate shows two inflection points as intensity rises:

• First Threshold (LT1 / Aerobic Threshold / Lactate Initial Rise Point): Blood lactate begins to rise steadily above baseline, around 1.5–2 mmol/L. This marks the point where fat oxidation’s contribution begins to decline and glycolytic contribution increases. The upper edge of Zone 2 training sits here.
• Second Threshold (LT2 / Maximal Lactate Steady State MLSS / Anaerobic Threshold): The highest sustainable balance between lactate production and clearance, typically 3.5–4.5 mmol/L (highly individual, so don’t rigidly stick to 4 mmol). Above this intensity, lactate and H+ accumulate rapidly, and exercise duration plummets to within tens of minutes.

Why Threshold Predicts Performance Better Than VO2max

Pace for a marathon or long time trial almost always falls between LT1 and LT2. If two athletes have the same VO2max (e.g., 70 mL/kg/min), but one has an LT2 at 85% of VO2max and the other only 75%, the former can sustain race intensity at a higher absolute output and will perform noticeably better. The threshold reflects the combined adaptations of mitochondrial density, capillary supply, and lactate transport proteins (MCT1/MCT4), all of which are more trainable than VO2max.

How to Measure It

• Laboratory Incremental Test: Increase load every 3–5 minutes and sample fingertip blood to plot the lactate curve. Stages of 4–5 minutes are recommended to reach a lactate quasi-steady state.
• MLSS Confirmation: Use a 30-minute constant-load test; if lactate rises <1 mmol/L over the final 20 minutes, it’s a steady state. This is the gold standard but time-consuming.
• Field Alternative: Functional Threshold Power (FTP) or critical speed can serve as a practical approximation of LT2, but they usually run about 3–7% higher than the true MLSS.

Training Implications

To push LT2 to the right, the most effective approach is “sweet spot/threshold” training at or slightly below LT2 (2–3 efforts of 10–20 minutes), combined with a large aerobic base below LT1. The polarized training model (roughly 80% low intensity, 20% high intensity, with deliberate reduction of threshold-zone work) has repeatedly been shown in elite athletes to raise both thresholds simultaneously.

Stop blaming lactate—it’s your body’s smartest fuel delivery service. The goal of training isn’t to eliminate lactate, but to teach your muscles, heart, and liver to recycle it on the spot at higher intensities.

Related Reading

• Lactate Kinetics and Buffering Capacity: The Lactate You Misunderstand
• Lactate Kinetics: An In-Depth Cycling Analysis
• Lactate Threshold Training Methods: The Scientific Breakthrough in Cycling Endurance
• The Truth About Lactate: It’s Not Your Enemy, It’s Fuel