Why "properly" feels right

If you believe that twenty minutes isn't worth bothering with, you didn't invent that. You were told it, and by good authorities. This chapter takes the belief seriously, because I think most people with this habit are trying to do the right thing by the best information they were given.

I can find five reasons behind the threshold.

The first is the old advice. For a couple of decades, public health guidelines said adults should get thirty minutes of moderate exercise on most days. They added that it could be built up in bouts, but that each bout had to last at least ten minutes to count. What most people took from this was simpler. Thirty minutes is the unit, and below some minimum it doesn't register. If you absorbed that, you were paying attention.

The second is the idea that the body needs time to get going. You've heard that you don't start burning fat until twenty minutes in, or that the first ten minutes are only a warm-up. On that view a fifteen-minute session stops before the useful part begins.

The third is sweat and effort. Real exercise leaves you red in the face and needing a shower. If you're not gasping you're not improving. A brisk walk in your work shoes doesn't feel like anything, so it can't be doing anything.

The fourth is the overhead. A gym session has fixed costs, which are travelling, changing, showering and changing back. If those take forty minutes, it makes no sense to pay them for fifteen minutes of exercise. That reasoning is sound as far as it goes.

The fifth is about standards, and it's the one I respect most. You're someone who does things properly. When you take something on, you don't do a half-hearted version and call it done. A ten-minute token effort feels like the sort of thing people do so they can tell themselves they've exercised. You'd rather be honest, and do it right or admit you didn't do it.

Underneath all five is a reasonable wish. You want the effort to be worth something. Nobody wants to go through the motions for no result. If it were true that short sessions did nothing, then skipping them would be the rational choice, and I'd have no book to write.

So the next several chapters take the question head on. Does a short amount of movement do anything? They look at what it does for how long you live, and for how you feel that evening. They look at whether it needs to be continuous, and whether it needs to hurt. On most of these questions the evidence is better than it is for most things in this series. Large numbers of people have been followed for many years, and in the recent studies their movement was measured by devices and didn't depend on memory. I'll say where the evidence has limits. The general direction isn't in much doubt.