Anaerobic Threshold (AT)
The intensity ceiling where fat fuel gives out
Plain English
The anaerobic threshold is the exercise intensity above which your body can no longer sustain energy production through aerobic (oxygen-based) pathways alone and begins accumulating lactate faster than it can clear it. Below this point, you can maintain effort indefinitely with enough fuel. Above it, you are on a clock. It is also called the lactate threshold 2 (LT2) or the ventilatory threshold.
The Mechanism
As exercise intensity rises, muscles increase their demand for energy. The aerobic system, which uses oxygen to produce energy from fat and carbohydrates, can meet moderate demand efficiently. But aerobic energy production has a ceiling determined by mitochondrial density and oxygen delivery capacity.
Above that ceiling, the body recruits fast-twitch muscle fibers that rely on anaerobic glycolysis, which produces energy quickly but generates lactate as a byproduct. Below the anaerobic threshold, working muscles and the liver can clear lactate as fast as it is produced. Above it, clearance falls behind production and blood lactate rises rapidly. This inflection point is the anaerobic threshold.
Physiologically, the anaerobic threshold corresponds to approximately 85-90% of max heart rate for most trained individuals and produces the distinctive breathing change that coaches recognize as the point where speech becomes fragmented. Exercise at or just below this threshold (often called tempo training) is highly effective for raising both lactate threshold and VO2 max, because it challenges the aerobic system at its upper limit without exceeding it for extended periods.
Why It Matters
VO2 max sets the ceiling. Anaerobic threshold determines how much of that ceiling you can actually use.
The anaerobic threshold is the most important performance variable for endurance sports. A runner, cyclist, or rower who can sustain higher power or pace below their threshold will outperform a competitor with the same VO2 max but a lower threshold. Training raises the threshold by improving both lactate clearance capacity and mitochondrial density, allowing you to work harder while staying aerobic. For recreational athletes, knowing and training near this threshold is the fastest route to improving sustained pace.
Common Misconception
Many people confuse the anaerobic threshold with maximum effort. The anaerobic threshold is a sustained, uncomfortable but manageable intensity, typically 20 to 60 minutes of hard effort, not a 30-second sprint. Training at true maximum effort is above the threshold; Zone 3 and tempo training sit right at or just below it. Sustained race pace for a 5K or 10K is close to anaerobic threshold for most trained runners.
What a Healthy Range Looks Like
Well Below AT
60-75% max HR
Zone 1-2: aerobic base building, fat oxidation, sustainable indefinitely with fueling
Approaching AT
75-85% max HR
Zone 3 gray zone: aerobic but harder; accumulates fatigue without the full AT adaptation benefit
At Threshold
85-92% max HR
Zone 4 tempo: AT training, sustainable for 20-60 minutes; highest aerobic adaptation per unit time
Above AT
92-100% max HR
Zone 5: VO2 max work; lactate accumulating; sustainable only for short intervals of 1-8 minutes
Anaerobic threshold is not a fixed number: it rises with training. Periodically testing your threshold through a 30-minute time trial (average HR in the final 20 minutes approximates AT heart rate) tracks progress over a training season. A threshold that creeps higher across months reflects genuine cardiovascular adaptation.
Signs It Is Disrupted
- Pace or power at threshold pace declining across training weeks despite consistent effort, suggesting accumulated fatigue depressing performance
- Breathing becoming labored at intensities that previously felt manageable, indicating threshold has dropped due to detraining or illness
- Heart rate rising more steeply than usual at a given pace or power, suggesting autonomic fatigue or early illness
- Recovery time between threshold intervals extending across a training block without a planned deload, pointing to accumulated fatigue
How to Improve It
3 Things to Remember
The anaerobic threshold is the intensity where lactate production outpaces clearance: below it, you can sustain effort; above it, you are on a clock.
Threshold training (tempo runs, sustained hard intervals) is the highest-leverage training tool for improving sustained endurance performance.
A higher anaerobic threshold lets you race faster while staying aerobic: it is the variable that separates equally-fit athletes at race pace.
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