alt Hacker NewsHIIT or Tabata must not be done day-to-day, everyday. With no recovery days, there will be no gains. A 21 year old is able to recover like crazy, a good 9 hours of sleep might be enough to mostly recover. For others it would not work well and would lead to over-training. In overtraining, the 100% efforts will be 90% efforts, impact the immune system, generally just not good all round.
For disproportionate benefits, one needs to define which benefits exactly. A max effort will burn a lot of calories quite quickly and potentially increase V02 max (which is highly correlated to overall longevity). Zone 2 training has become popular and has other benefits, notably increases 'fat max' threshold - which gives different benefits (specifically the ability to work harder for longer while still using fat as an energy source for the exercise).
> I found that it's indeed difficult to strike a legitimate 100% effort, even when it's only for 20 seconds and only 5-10 times.
This is essentially the point. At the end of Tabata, the last interval should be the last bit of energy you have in the tank. It should be entirely draining. Doing this routine daily will not allow recovery to then properly do the training well.
FWIW, I heard it paraphrased as this: the body has essentially too modes, hard & easy. When going hard, it only matters how hard you go, not how long. When going easy, it only matters how long you go for, not how hard. At the same time, zone 2 training and HIIT/Tabata are not mutually exclusive in their benefits, but it's more which systems receive the most benefit while other systems in the body receive benefits but to a lesser degree.
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> With no recovery days, there will be no gains.
That is true of any exercise regime with much intensity. For muscular activity: pushing towards anything like your limits technically causes lots of minor damage, which the body repairs back better. If you don't give yourself sufficient recovery time within your weekly routine you miss out on a lot of that benefit because the body's repair/improve systems don't have time to properly do their thing. This is one of the reasons¹ why overtraining injuries are a thing. In terms of cardio this still applies, the heart is a set of muscles. Mentally I think there is a similar effect, but pinning down a cause for this is much more hand-wavy and subjective compared to the far better understood² mechanisms of how the body repairs, regulates, and improves, physical structures.
Some people seem to manage with minimal recovery time, but they are either lucky³, kidding themselves, or storing up issues ready for a big nasty surprise later.
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[1] Other similar reasons include damage & fatigue in parts of the body other than muscles, and sometimes just being tired to the point of getting form wrong and hurting yourself through that.
[2] though still not entirely understood
[3] I'm counting being young as being lucky here. I'm trying to get back into running and other regular exercise (after a period of illness, looking after family with medical issues, and general burn-out) and the biggest thing getting in the way of improving from here is that I'm now in my mid/late 40s rather than early 30s like last time I was at this level of conditioning!
The original experiment was four days per week of the "exhaustive intermittent training" and a fifth day was 30 minutes of zone 2. That's what I followed. It does seem like that fifth day has been forgotten when people talk about Tabata. Like you said, "zone 2 training and HIIT/Tabata are not mutually exclusive," and I've gotten my best results when doing a few hours per week of zone 2 running with a dash of higher-speed intervals or repeats one day.
Now that I'm quite a bit older, despite maintaining my body weight and two-mile running time since then, I'd probably get hurt if I repeated the experiment.