https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3912323/
In a nutshell polarized training is going really hard 5-10% of the time and really easy the other 90-95% of the time, unlike what most actually do which is moderate effort all the time regardless of how they perceive their efforts. It's just putting a fancy name to the age old concept of going hard when it's time to go hard, and not thwarting those hard efforts by going hard when it's time to go easy. We can poke all sorts of holes in the methods in the study, for example properly structured HIIT is polarized training. A good takeaway is that if you get discouraged because you don't have time or energy to do hard conditioning multiple times a week, you don't need to do that. If you're killing yourself in the gym, and your worn out and miserable, you can dial it back a notch.
Take HIIT for example. The vast majority of the experts on the interweb love to misquote Tabata, and get all wrapped up around the axle about the rest intervals. If you try to replicate the Tabata protocol on a Bike erg and use a HRM, it's just high intensity training, there's no actual recovery between intervals, and frankly that protocol was developed in the glory days of EPO. Anyways, the goal of High Intensity Interval Training should be high intensity intervals with as much rest as needed between sets to produce high intensity intervals, not moderate to low intensity intervals with short rest periods. The high intensity produces the results, not how closely they are crammed together. On a larger scale, there needs to be enough time to recovery from the workout before doing it again.
The concept applies at micro and macro levels of training.
Tabata study
https://www.researchgate.net/publica...ity_and_VO2max