The issue is that Z3 generate fatigue metabolites whereas Z2 doesn’t, and the benefits of Z3 above Z2 are limited for many people, so better to do the Z2 hour, and then the next day be able to better perform a HIIT session. I personally don’t think that less than 12hr/week doesn’t need specificity as most of the people I coach do below that, and almost always find that tailoring the training and doing specific efforts alongside Z2 work is the best for them. It’s also why we have adjusted the SYSTM plans to account for the knew knowledge presented to us in research and also feedback from our many users