Norwegian 4x4

Norwegian 4x4
https://pmc.ncbi.nlm.nih.gov/articles/PMC7399937/

The Norwegian 4x4 protocol is a HIIT method that is an extensively researched program that was developed by the Norwegian university of science and technology.
Studies show that performing this 2-3x per week can improve V02 max by 7-8% in 8 weeks.

Consisting of 4 rounds if 80-95% max heart rate followed by 3 minutes of active recovery.