Zone 2 is the effort zone between the first and second lactate threshold (blood lactate 2-4 mmol/L). The definition is reasonably solid, but whether this zone is optimal to train in is an open question: elite athletes actively avoid it, and the mechanistic benefits have not yet been proven experimentally.
Zone 2 is a specific training intensity zone defined as the effort between the first and second ventilatory or lactate threshold. In practical terms this corresponds to a blood lactate value between 2 and 4 mmol/L. You are working harder than an easy endurance run (zone 1), but you have not yet reached the intensity of high-effort training (zone 3). A well-known rule of thumb: you can still hold a conversation, but it takes effort.
In the literature, zone 2 is sometimes referred to as the 'grey zone' or 'lactate accommodation zone'. This points to an intensity that is hard enough to be fatiguing, yet at the same time not hard enough to deliver the strongest training stimuli. Whether that is an advantage or a disadvantage is precisely the point of debate among researchers.
Interestingly, elite endurance athletes actually spend very little time in zone 2. Research involving well-trained junior speed skaters showed that only 6 to 8 percent of their training fell within zone 2. The vast majority of their training is in zone 1 (approximately 75 percent) or in the high-intensity zone (15 to 22 percent). This pattern is called a polarised approach: the deliberate avoidance of the intermediate zone. For recreational athletes or people training for health benefits the situation may be different, but that cannot be stated with certainty on the basis of the available claims.
Proponents of zone 2 training point to two possible advantages. First, training at this intensity is said to allow faster recovery than intense exercise, because central and peripheral fatigue remains lower. This would make multiple sessions per week feasible. Second, this intensity is thought to stimulate the production of new mitochondria (the energy factories inside cells) via two biological signalling pathways: calcium and AMPK, a kind of energy sensor in the cell. Both explanations, however, are based on theoretical models and have not yet been directly demonstrated in randomised experiments, so caution is warranted.
In short: zone 2 is well defined as an intensity zone, but whether it is the best or most effective training zone remains a matter of debate. Elite athletes appear to actively avoid it, while certain training models regard it as a key element. The evidence is, for now, associative and partly theoretical in nature.
Based on three PMIDs (35418513, 36900796, 16430681, 32149880). The definition and distribution are moderately supported by observational data from elite speed skaters. The mechanistic claims (mitochondria, recovery) come from a single source using a theoretical model, with no RCTs.