HIIT is not endurance training for elite athletes

It's been a month since I posted, and despite being quite busy, I'm going to type into the internet void.  High-intensity-intervals-as-endurance-training is all over my usual internet haunts, specifically r/science and r/velo.

As usual, the link goes to a news article rather than the study, though the study is linked at the end of the article--a practice I wish were more common.  From the article:

Short bursts of just a few minutes of exhausting physical activity [boost] the production of new mitochondria...which culminates endurance enhancement much like more time consuming endurance training.  High-intensity exercise triggers the breakdown of calcium channels as a result of an increased production of free radicals. The muscle cells thus have anti-oxidative systems for trapping and nullifying the radicals.

Moving backwards, all cells have methods to nullify unwanted radicals, but let's recall that radicals are not necessarily bad.  Here's a primer that's long and excellent.  So the ryanodine receptor type 1 (RyR1) and sarcoplasmic reticulum (SR) are disrupted by HIT, which causes radical generation that the cell recognizes as oxidative stress.  In turn the cell makes more mitochondria.

Popularly, this is hailed as hard evidence that HIT is just as good as low intensity training.  An astute observer from r/velo notes: "To me this reads like, train like a track sprinter and become the next TT world champ."  Indeed.  Clearly that never happens, so what's going on?

All we really need to know from the study to answer this question is in the abstract, which is publicly available.  It states that the main study was done on moderately active individuals, and that in elite endurance athletes "the measured transcript levels related to mitochondrial biogenesis and endurance showed a general decrease— rather than the expected increase—24 h after the HIIT exercise."  The explanation for this is simple.  Experienced athletes spend a good deal of time activating their muscles, and any easily fracturing RyR1 or leaking SR would be some of the first adaptations that take place in order to facilitate more exercise.  Because let's face it, if your calcium machinery isn't working properly, you're going to have serious problems with muscle contractions.  To confirm this, another quote from the abstract: "the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes."