Can you really do HIIT on a treadmill? It doesn't sound safe to sprint on a machine.
Re your observations. Are you referring to elderly people? I'm not sure this study is about them. My mental model of this mechanism is that relatively young people can choose between e.g. long distance running and sprinting, and that the latter will yield better cardiac outcomes.
The kind of HIIT I'm doing is only 10-20s bursts, to stay within ATP-PCr energy metabolism (little lactic acid production). A short sprint followed by 2 minutes rest. That's hard to achieve with a treadmill.
Re your observations. Are you referring to elderly people? I'm not sure this study is about them. My mental model of this mechanism is that relatively young people can choose between e.g. long distance running and sprinting, and that the latter will yield better cardiac outcomes.