We go deep into a couple papers that measure the relative contributions to early VO2max improvements, and the evidence about whether they're more are muscular or cardiac in nature, and what physiological differences there are with more well trained people. Moderate and high intensity training are contrasted, as well as the obvious shortcuts, plus a first-principles approach to alternative mechanisms. We also answer your listener questions on if you can screw up newbie gains, how much is just mental toughness, and more.