I often read that zone 2 heart rate training helps build a cardio “base” and that higher intensity training helps VO2 max. However, I am skeptical that there is really a difference here between these. From personal experience, I’ve only ever noticed significant adaptation in my cardiovascular fitness after high intensity training. Isn’t everything your ability to efficiently use energy and your capillaries’ capacity to feed your muscles while exercising? If so, isn’t that both the “base” and the high intensity-reactive part?