Is Base Training the Only Training That Improves Mitochondrial Function?

The hallmark of base training is relatively large volumes of low intensity riding or running depending on your sport. What's actually happening inside the body to cause those changes and then how are those structures changing to accommodate the improved performance? This is what I've seen over the years. We don't have solid research to show this, but we have a lot of data which implies that there's a big improvement at the mitochondrial level. Those ones who do this volume for months, right at that zone two, if you will, and then some intensities here and there, but mainly the zone two, they have the highest improvement in fat oxidation as well as lactate clean capacity.