One thing that I think gets looked past in studies like this is the “noob gains” effect. These participants are healthy adults but not highly trained. It’s pretty well known, in cycling at least, that hard interval training is super effective for untrained people or people coming back from a break, but the gains plateau relatively quickly, and the stress of doing this kind of work is hard to sustain for long periods. Another notable thing is that they are doing 45 minute sessions regardless of intensity, in the real world it’s common for lower intensity sessions to be longer, and for those sessions to be a foundation on which higher intensity sessions are carefully added.

I've read a looot of physiology studies about running, and the same dynamics hold true there. This is a flaw (IMO) of of the whole "Norwegian 4x4 protocol" you hear parroted around by biohacker/longevity influencer online, of course with 0 nuance or context of the underlying study and its limitations

This study followed everyone for 6 months so they probably got past that factor.

> It’s pretty well known, in cycling at least, that hard interval training is super effective for untrained people or people coming back from a break, but the gains plateau relatively quickly, and the stress of doing this kind of work is hard to sustain for long periods.

This is 100% experience with both cycling and running, and something I worked out on my own early on, prior to the advent of smartphones and even talking to anyone who knew anything.

I enjoy sprinting, both running and cycling, but it’s mostly something I do to regain my endurance ability after a break. Two two weeks of high intensity interval training, and then I’m able to sustain moderate intensity jogging for 30+ plus again, or an hour cycling.