Anaerobic thresholdVentilatory threshold

The anaerobic threshold is the effort level at which the aerobic system can no longer keep up with demand and anaerobic energy production rises steeply. It is measured in the lab through breathing changes (the ventilatory threshold) or through blood lactate, where it overlaps heavily with the lactate threshold.

It matters because it marks the practical ceiling of sustainable effort far better than maximum heart rate or VO2 max do. Two runners with identical VO2 max but different thresholds will race very differently, because the one who can hold a higher fraction of their maximum aerobically will hold pace for longer.

In practice the term is used loosely and often interchangeably with lactate threshold. The distinction that matters for training is not the label but the fact that this point is highly trainable — it moves upward with consistent aerobic volume and threshold work, more readily than VO2 max does.