The Norwegian 4x4 protocol, involving four 4-minute high-intensity intervals with 3-minute recovery periods, is a highly effective method for improving VO2 max.

“I like to talk about probably one of the hardest ones. Uh it's called the Norwegian 4x4. This the reason it's called that is because the Norwegian ski team uses it as part of their training protocol. And what it is is you for 4 minutes. This a 4-minute interval where you're going as hard as you can, not all out, as hard as you can and maintain for that entire four minutes. And it takes a little bit of trial and trial and error to figure out what that intensity is. It's going to be it's going to be hard, but you're not going all out. that that would be impossible. And then you and then you do light exercise, very very light exercise. You let your heart rate go down and recover for 3 minutes. And you do that four times. So it's four by four. And um this protocol has been shown in countless studies to improve the O2 max.”