Aerobic Threshold (AeT)

At low exercise intensities, fat is the dominant fuel and the aerobic system handles nearly all energy production. As intensity rises, carbohydrates begin contributing more, and muscle cells start producing small amounts of lactate. The aerobic threshold (LT1, or the first lactate threshold) is where this transition becomes measurable: blood lactate first rises above resting baseline, typically to around 1 to 2 mmol/L. Below LT1, training is predominantly fat-burning and aerobic. Above it, carbohydrate reliance increases, and effort shifts toward the metabolically mixed Zone 3.

The aerobic threshold corresponds to the top of Zone 2 heart rate for a given athlete. In the field, it is identified by the breathing pattern: below AeT, a person holds a full conversation without effort; above it, speech becomes slightly labored. In a lab, it is measured via blood lactate testing or a metabolic efficiency test, in which fat oxidation peaks at AeT and declines as intensity rises. Research by Philip Maffetone and others identified AeT as the primary training boundary for building aerobic efficiency in endurance athletes.

Most recreational athletes train habitually above their aerobic threshold without knowing it, spending significant time in Zone 3 (above AeT but below LT2). This "moderate-intensity trap" delivers weaker aerobic adaptations than Zone 2 and weaker threshold gains than LT2 work, at a higher recovery cost than either. Over weeks, this pattern keeps the aerobic base underdeveloped while accumulating fatigue, limiting progress despite consistent training effort.