My reading about Polarized is that the low intensity needs to be really low and lots of it throughout the whole plan, and the High needs to be really high, and up to 20%., That said some articles also suggest a small component of something inbetween, but that is very small , eg, 5% max.
Both the medium and high volume plans follow a similar structure for the HIIT workouts.
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One workout is essentially repeat 2 minute intervals with 3-4 minute active recovery; these increase over the 8 wks, either in target FTP intensity 120%+ or reduced recovery. In the second block also duration bumps up a lot from 1 hour to 1.40.
The second workout is essentially repeat long intervals, typically 16mins, close to FTP; their duration also ramps up to 2 hours.
In the Medium plan 20% HIIT would be between 66 and 116min weekly total for both sessions. In the long plan 104-140min. But the pans are significantly higher, both in the 80-220 min range. Overall in the Low 31% in the long 23%, and if you remove the adaption weeks that % increases significantly.
So two things seem questionable.
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The weekly duration of the HIIT workouts is well beyond what the proponents and research into Polarized recommend, ie, up to 20% HIIT
Secondly, the 16min at 100% FTP also doesn’t seem to match the research., ie, too long and too low intensity
My reading suggests shorter workouts at the higher intensity. Hence VO2Max interval sets typically 2-5 minutes plus long recovery at very inactive levels. Secondly Anaerobic Needle type sets of a few seconds, up to 30sec at very high intensity with relatively long recovery at very inactive intensity.
Where all the R&D is at its vaguest is how they fit and change within a periodized plan and its phasing, base, build and specifics.