I’m curious as to what metrics you are using to determine the profitability…
Caveat: So, for me, I'm starting at a relatively low level of cardio/conditioning, anyways.
I've tried including some things and they weren't quite on target for me, loaded carries, step ups, etc. they didn't hurt, but they were not exactly what I was looking for, apparently; cardio-wise.
That said, ...
There are a few changes I've noticed since I started...
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Blood pressure
I can sense my own pulse to one degree or another, and the intensity of that sensation has been lessening in intensity. It's most apparent after bounding up a flight of steps. Bounding up a flight of steps at home is one of my favorite tests, since I have a stair case at home, I can just run up it as fast as I can, to see if it was easier or harder than I remember.
Also, I have been pre-hypertensive in recent years when they take my vitals at a check up. Kinda just above the normal range. I wonder if that'll have changed next time it's measured.
Step count
I think the first time I tried these 20 second walking sprints I counted ~28 steps. That went from 28 steps to 30, to 32, to 35, and now Today my fastest sprint counted 38 steps. Several of them were solidly 37 step efforts.
I count the foot falls of the same side foot. So, each time the left foot steps, (left)1, (right), (left)2, (right), etc.
RPE
Just as I've been observing the fall in RPE from my AXE training, so too have there been decreases in RPE from these short sprint sessions.
Yeah, I'm breathing hard when I finish but not quite as hard as when I first started. My heart is quickly pounding, but not quite as much as when I first tried this.
A set of burpees is not quite as taxing as before. The speed walking sprints are not quite as difficult.
What the hell effect?
I found it to be interesting that my RPE had a drop for my loaded carries when I started including these sprints. That's something I want to look at again later: Whether the direct cardio training more effectively improves my loaded carry performance (maybe just for now, during something like a newbie gains period.) than loaded carries, practiced regularly.
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So, as far my training is concerned, a minute or two a day does a lot for me because of my lower conditioning levels, and aspirations. The most I have to do is carry one of my kids a long way, or move some branches and trees around on the property.
An aside
But, as Dr Gibala's book points out, Mr Andy Magness apparently used HIIT training for longer duration events like ultra marathons or Iron Man races.
Link if interested to Mr Magness book. "Ultra Mental"