Anaerobic thresholdVentilatory threshold
The intensity at which energy production shifts sharply towards anaerobic pathways.
The anaerobic threshold is the intensity above which effort becomes unsustainable relatively quickly, because energy production leans increasingly on pathways that accumulate metabolic by-products faster than they can be cleared.
The name is a historical misnomer worth knowing about. Muscle does not suddenly switch to oxygen free metabolism at a threshold; aerobic metabolism continues and is supplemented. That is why the literature increasingly prefers terms such as the second lactate threshold or critical power.
The terminology is genuinely chaotic beneath the concept. A major review catalogued a long list of competing definitions and calculation methods, all called something similar and none interchangeable, which is why two tests can hand you two different numbers and both be defensible.
It matters because it predicts endurance performance well, often better than VO2max among trained athletes, and because it defines the boundary between training you can accumulate and training you must ration.
Estimating it does not require a laboratory. The intensity you could hold for roughly an hour, the point where full sentences become difficult, and heart rate bands derived from a maximum estimate all land close enough for training purposes.
It is trainable from both directions. Large volumes of easy aerobic work improve clearance capacity and the machinery underneath, while threshold and interval work raise the intensity at which the crossover happens.
In practice, treat any single threshold number as approximate, judge progress by the pace you can hold at the same effort, and set your bands with our heart rate zone calculator rather than chasing precision you cannot verify.