My point was merely that I simply do not believe that you can get away with a much lower average intensity, spend the same amount of training time a week as with a higher average intensity protocol (like HIIT) and still somehow get the same effect. It doesn't make sense to me from a stress-adaptation perspective.
Ok, now I understand better what you're saying
Nobody is doing HIIT 6 times a week
I did.
Crossfit 5x per week. Monday through friday with rest on weekends.
I came in earlier and did mobility, then 15min on the rower and 531 for my strength training (Mon, Tue, Thu, Fr, nothing on Wed).
After that the complete hour of crossfit which usually was 10min warm-up, 10-15min strength work (I skipped this, because it was unstructured compared to SS, 531 etc. and did some skill-work instead), followed by the met-con of the day and finished with cooldown + stretching.
I went 100% on every metcon, because we had a very competitive enviroment in our box.
I had some minor issues from poor exercise execution under fatigue, but I did this schedule for a year without any signs of overtraining. I felt beat down for 20-30min after the met-con, but afterwards I felt great and energetic.
Without any running, besides the occasional 400m, 800m or 2k build into a met-con my 5K went from ~25min to a 19min within 7 months.
After I quit crossfit, because I had to move cities I went to a new gym (non-crossfit) and they did a test at the beginning, which was done on a stationary bike. I scored a 10/10 on their chart and this is the gym that's responsible for the S&C for the local volleyball team that plays in the highest national league and is good enough to compete for the championship.
Yes, I was 23 back then and could recover from much more than e.g. a 40 or 50 year old, but it still shows that you can handle tremendous loads of work and high intensities without breaking down.
You don't have to do strength training followed by intense met-cons 5x per week, but even at 50+ you can still recover fine from 3-4x HIIT per week.
To give an example, I think
@Bill Been is over 50, trains HIIT + strength and is doing fine.
@305pelusa to support your point, I did 5x HIIT met-cons per week for a year and S&S (by the book) 5-6x per week for more than 6 months. S&S beats HIIT in several areas (at least for me and my body), but when we just talk about the cardiovascular improvements (aerobic + anaerobic, e.g. 5K time, resting HR) S&S doesn't even come close to HIIT (again in my case, YMMV).
Of course I can't tell you whether my mitochondria were doing better on one or the other method.
I'm still young and already see that I can't handle the same amount of punishment like I could do 5-10 years ago. I believe in people like Al Ciampa, who tell me that in the long run something like A+A is better for me and my health, because he trains people for several decades now and has seen first hand what effect X type of training methodology has on people over time.
As a sidenote, I wouldn't recommend crossfit because of their exercise selection. High rep olys, boxjumps etc. under fatigue are simply stupid. It was things like that which lead to my minor injuries. The mentality is good though. If you were to do all that high intensity stuff e.g. on a stationary bike, the chances of injury via poor exercise execution would be minimized. Also your fitness (endurance) improves a lot, but you aren't really getting stronger, but all of that crossfit pros/cons is a different topic...