Crucially, they could still accelerate to faster speeds; however, their oxygen intake no longer followed. This plateau is your VO2max, a pure and objective measure of endurance capacity that is, in theory, independent of motivation, weather, phase of the moon, or any other possible excuse. Hill surmised that VO2max reflected the ultimate limits of the heart and circulatory system—a measurable constant that seemed to reveal the size of the “engine” an athlete was blessed with.
