The faster you run, the harder you breathe. Also, the longer you run, the harder you breathe. And if you sustain a fast running speed long enough, you will eventually find yourself breathing as hard as you can. Exercise scientists have a special term for breathing as hard as you can: VO2max. Whatever you choose to call it, running at this intensity level is rather painful. If you do it right, though, it is extremely beneficial. In fact, minute for minute, VO2max training boosts running fitness more than any other type of run training. Before I give you some tips on how to do VO2max training, let me first explain the physiological rationale behind it.