Or there's the Norwegian 4x4. This is one of the best ways to improve your VO2 max. And that is where you go 4 minutes at an intensity that's pretty high, that you can maintain and sustain for that entire 4 minutes—usually it's about 85% max heart rate—so you're going pretty hard for the entire 4 minutes, as hard as you can maintain for the entire 4 minutes, and then you rest for 3 minutes ... So you do it four times. And that is one of the best ways to improve your cardiorespiratory fitness.