"It's my age"

The age story deserves to be taken seriously, because part of it is true. Bodies do change with the years. Muscle gets harder to keep, recovery slows, and joints that have done fifty years of work show it. If I told you that age had nothing to do with how you feel, you'd be right to stop reading.

The question is how much of what you feel is age and how much is disuse. For most people the two have arrived together, so there's been no way of telling them apart. The more years you've lived, the more of them you've spent at a desk. You'd need an experiment that separates the two. There's a small, famous one that does.

In the summer of 1966, researchers in Dallas took five healthy men of twenty and put them to bed for three weeks. They weren't ill. The point was to see what complete rest does to a fit young body. The researchers measured each man's maximal oxygen uptake before and after. That's the most oxygen your body can take in and use when it's working flat out, and it's the standard laboratory measure of fitness. After three weeks in bed it had dropped sharply.

Thirty years later, Darren McGuire, Benjamin Levine and colleagues found the same five men, now fifty, and measured them again. Over three decades of ordinary adult life their maximal oxygen uptake had fallen by 11 per cent from where it stood in 1966. That was a smaller fall than three weeks in bed had produced when they were twenty. The researchers' own summary was that three weeks of bed rest had a more profound effect on the men's physical work capacity than three decades of ageing.

The team went back again when the men were sixty. By then the total decline across forty years had reached 27 per cent. The drop after those three weeks in bed in 1966 had been 26 per cent. Forty years of living and three weeks of lying still came out about level.

I don't want to make this small study carry more than it can. It was five men. Bed rest is a far stronger dose of stillness than a desk job. Age did show up in the numbers. The decline between fifty and sixty was about four times as fast as between twenty and fifty, partly because some of the men had developed medical conditions by then. And over those thirty years they'd put on a quarter of their body weight and doubled their body fat, so the ageing being measured was the ordinary sedentary kind. That last point cuts both ways, and I'll come back to it in chapter 13, because the researchers then did something interesting with the men at fifty.

What the study shows is scale. People assume that age is the big force and inactivity a minor one. In these men it was the other way about. Complete stillness did in three weeks what age needed decades to do.

It isn't only the young who respond that fast. Patrick Kortebein and colleagues asked eleven healthy volunteers with an average age of sixty-seven to stay in bed for ten days. Nothing was wrong with them, and they were fed properly throughout. At the end of the ten days the strength of their thigh muscles was down by 13 per cent, their stair-climbing power was down by 14 per cent and their aerobic capacity was down by 12 per cent. One further detail matters for this book. After the ten days were over and they were free to move as they liked, they moved less than they had before.

Your body runs on a simple policy, and I'm giving you my interpretation here and not a finding. It maintains what gets used and lets go of what doesn't. Muscle, heart capacity and the easy range of a joint all cost something to keep up, and a body that's asked for very little of them stops paying for them. It works the way a council treats a bus route. If nobody rides it for long enough, the service gets cut, and nobody at the council thinks of that as damage.

Now go back to the thirteen hours. A day like that isn't bed rest, but it's a diluted version of the same instruction, given daily for years. Your legs are asked to carry you for a few hundred yards in total. Your heart is never asked for more than it needs to keep you upright in a chair. The body draws the reasonable conclusion.

So when you stand up at twenty to seven and make that noise, the honest account is that two things are going on. One is that you're fifty-four. The other is that you've been still since five o'clock, and largely still for years. You can't do anything about the first. The rest of this book is about the second, which I think is the larger share, and which works in both directions.

One check you can run on yourself. Think of a time when you were forced to be far more active than usual for a week or two. It might have been a walking holiday, a house move, or a fortnight helping someone decorate. Most people remember being sore for three days and then, by the second week, feeling looser and livelier than they had in months. If what you feel on the sofa were age alone, that fortnight wouldn't have made any difference to it.