At Zone 2, mitochondria are at metabolic equilibrium and at the highest operating capacity. This is why it is a challenging intensity for well-trained athletes (it must be). Lactate is being produced at a meaningful rate as glycolytic rate is moderate, but the mitochondrial lactate oxidation complex (mLOC) is clearing it at the same rate, converting it back to pyruvate and feeding it directly into oxidative phosphorylation. The lactate shuttle, both within and between cells and tissues, is running at full capacity. Fat oxidation is at its peak. The NAD⁺/NADH ratio is stable, reflecting a healthy redox environment in which glycolysis and mitochondrial processing remain tightly coupled. The system is maximally stimulated while still in balance. This is precisely why Zone 2 is probably one of the most powerful sustained training stimulus for mitochondrial adaptation. Mitochondrial biogenesis requires that the organelle be pushed close to its capacity. Zone 2 does that, while keeping the internal environment stable enough that the stimulus can be sustained for a prolonged period and recovered from it efficiently. -And with all this said, high intensity training is absolutely key for performance. substack.com/@inigosanmillan…