Norwegian 4x4
Four hard intervals. The most studied recipe in cardio.
The protocol
The Norwegian 4x4 is a structured interval session: four minutes of hard effort at roughly 85 to 95 percent of max heart rate, then three minutes of easy recovery, repeated four times. With a warmup and cooldown, the whole session fits inside 40 minutes.
Why it's the flagship
Few training methods have been studied as thoroughly. The 4x4 is one of the most reliable ways to raise VO2 max, which is among the best-measured predictors of long-term cardiovascular health. It works on a treadmill, a bike, a rower, or a hill outside.
How hard is hard
Each four minute interval should feel challenging but repeatable. If you can't finish the fourth round, the first ones were too fast. If you could hold a conversation, they were too slow. The three minute recoveries are genuinely easy on purpose.
4 x 4 minutes
Hard intervals at 85 to 95 percent of max heart rate.
3 minutes between
Easy recovery, easy enough to feel almost lazy.
1 to 2 times a week
Enough to move VO2 max alongside the rest of your training.