MitochondriaMitochondrion

Mitochondria are the organelles where oxidative energy production happens. Fuel from carbohydrate and fat is processed through the Krebs cycle and the electron transport chain, using oxygen to generate the overwhelming majority of the cell's ATP. They carry their own small genome, a relic of their bacterial ancestry.

It matters because mitochondrial density and quality in skeletal muscle are among the most trainable determinants of endurance. Aerobic training increases both their number and their function — mitochondrial biogenesis — which is a large part of why the same pace gets easier over months of consistent work.

In practice this adaptation comes disproportionately from accumulated easy aerobic volume rather than from hard intervals alone, which is the physiological case for zone 2 work. Mitochondrial decline is also a recognised feature of ageing, and exercise is the intervention with the best evidence against it.