What is zone 2 training?

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 training 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 also referred to as the 'grey zone' or 'lactate accommodation zone'. That name points to an intensity that is hard enough to be tiring, yet at the same time not hard enough to provide the strongest training stimuli. Whether that is an advantage or a disadvantage is precisely the point of debate among researchers.

Notably, elite endurance athletes actually train very little in zone 2. Research involving well-trained junior speed skaters showed that only 6 to 8 percent of their training fell in 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 differ, but this cannot be stated with certainty on the basis of the available data.

Proponents of zone 2 training point to two possible advantages. First, training at this intensity would allow faster recovery than hard exercise, because central and peripheral fatigue remain lower, making multiple sessions per week feasible. Second, this intensity could stimulate the production of new mitochondria, the energy factories of 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 confirmed in randomised experiments, so for the time being they should be regarded as biologically plausible hypotheses, not as proven benefits.