The 4x4 interval workout (four minutes at 95% max effort, followed by three minutes recovery, repeated four times) is a high-intensity training protocol that can be judged by perceived exertion if a heart rate monitor is unavailable.

“A popular variation of this is called the Norwegian 4x4 interval training protocol so the intervals are 4 minutes long and you're aiming for about 85 to 95% of your max heart rate or the maximum level of intensity you can maintain for the entire four minutes um these interval these intervals can be brutal so the recovery period is 3 minutes long and the intensity is significantly lower like light light exercise more like a Zone one training exercise you want to allow yourself recovery time the clearance of lactate you want your heart rate to come down significantly so that you can prepare for the next 4minute interval so these four minute intervals are repeated um four times and again in between each interval is a 3 minute recovery so that's the the nuran 4x4 interval training”

“Jens Hoff's uh 4x4 which is the old Norwegian ski teamwork out four minutes at 95% of Max followed by three minutes of recovery repeated four times even if you don't have a heart rate monitor on it's basically as hard as you can go for 4 minutes and at the end of that 4 minutes you need to be ready to stop and then at the end of the 3 minutes of recovery you need to be ready to go again and that's how you judge that intensity completely independent of heart rate”