Reading the study, participating in our was quite commitment. No wonder people declined.

HIIT is relative to your shape, so their workout was not the same as 24 years old dude in shape would have.

Still, going at 90% of your maximum heart rate repeatedly for two-three minute intervals is really hard work, at any age and shape.

And as others pointed out, the baseline group here is probably a force of nature unto themselves. 75-year old Norwegians were born into a poor country just starting to recover from war and occupation, where most people were farmers or fishermen. It was not until these people had been working adults for 10 years that Norway discovered oil and began its rapid economic growth, by that time these people were used to hard work.

Then you take that generation, and select the toughest portion out of those who say they are interested in participating in a scientific study on exercise, so a two stage selection of fit people.

Basically the baseline here consists of people who think hiking in the mountain for several hours is great fun, and that it's been a bad week if they have not done so twice.

> Basically the baseline here consists of people who think hiking in the mountain for several hours is great fun, and that it's been a bad week if they have not done so twice.

Isn't it like, almost everyone? Except possibility to do it twice a week. Most people I know think hiking in the mountain is great way to spend holiday and they are not particularly tough.