What the test measures

VO2 max is the most oxygen your body can use per minute, divided by your weight. We measure it on a treadmill with a mask, raising the gradient every minute until the number stops climbing. It takes about twelve minutes and the last two are not pleasant.

The result is in millilitres per kilo per minute. A sedentary man of fifty usually reads between 28 and 33. A trained one reads 45 or more. For women the same bands sit roughly 15 percent lower.

What it predicts

In large cohorts that tested people on a treadmill and followed them for a decade or more, fitness separated the outcomes more than smoking, diabetes or blood pressure did. People in the bottom quarter died at roughly four to five times the rate of those in the top band. Moving from the bottom quarter to the one above it accounted for most of that difference.

The largest gain is not from good to elite. It is from low to average.

That is the part worth repeating. You do not need an athlete's number. You need not to be in the bottom quarter for your age.

What it does not predict

VO2 max is an association measured in populations. It does not tell you whether your arteries carry plaque, whether your blood sugar control is slipping or how well you recover. A runner with a VO2 max of 52 can still have an ApoB of 130.

It is also partly inherited. Two people who train the same way for a year can improve by very different amounts. A modest gain from a low starting point is worth more than it looks.

Why wrist estimates are not enough

Watches estimate VO2 max from heart rate and pace. For a steady runner on flat ground the estimate is often within 10 percent. For anyone who cycles, swims, walks hills or takes a beta blocker, the error can be much larger. We use the watch for the trend and the treadmill for the number.

How we use it

• A baseline test at the first visit, with heart rate zones and lactate from the same run.
• A training plan built on those zones: three or four hours a week, most of it easy.
• A repeat test at six and twelve months, in the same conditions and at the same time of day.

Member 0287 started at 31, the bottom quarter for a man of 54. Twelve months of easy aerobic sessions and one interval day a week brought him to 38. That is not a remarkable number. It moved him into a different risk band, which is the point.