How to improve VO2 max: train your engine in intervals.

VO2 max, your maximal oxygen uptake, is the most oxygen your body can use during hard effort: the size of your engine. To improve it, you need plenty of easy cardio plus, once or twice a week, four four-minute intervals at a hard pace.

It's Monday, before your eight o'clock call, and your watch tells you your VO2 max has dropped a point. I see it often in the managers I work with: they check the number every day and always train at the same pace.

What is VO2 max, and what's a good value?

It's your aerobic ceiling. It depends on how much blood your heart pumps with each beat and how much oxygen your muscles take up, measured in millilitres per kilo per minute.

• The key step. At rest you use about 3.5 millilitres, which is one MET. Eight METs, about 28 millilitres, is roughly what you need to run at 11 km/h for a minute. Below that, the risk of death rises.
• Values by age. At fifty, a theoretical estimate gives about 30 for a man and 25 for a woman. It peaks around twenty, then slowly falls.
• Why it matters. Each extra MET comes with more than a ten per cent lower risk of death. Poor fitness can count for more than high blood pressure, high cholesterol or extra weight.

How accurate is the VO2 max on your watch?

Good enough to follow the trend, not to trust a single day. A wrist device makes an estimate, with a margin of error. The real measurement is done in a lab, flat out, with a mask measuring your oxygen.

Watch the month, not the morning. One point down after a week of flights means little; four weeks of decline is a signal.

Why you need minutes near your maximum. VO2 max rises when you work close to your ceiling, at a pace you could hold for five or six minutes at most. That's where your heart learns to pump more blood per beat and your muscles to take up more oxygen.

How can you improve VO2 max after 40?

With a mix: roughly eighty per cent of your cardio at an easy pace and the rest hard. I cover the easy base in the article on zone 2.

Month one: short intervals. Once a week, fifteen minutes alternating one minute hard with one or two easy. The first three or four weeks feel the toughest; then your body usually responds.

Then the four-by-four (4x4). Ten easy minutes to warm up, four four-minute intervals at 90 to 95 per cent of your maximum heart rate, three minutes' active recovery in between, and five to cool down. Your maximum is roughly 220 minus your age: at fifty, aim for 153 to 162 beats. On a bike, a rowing machine or walking uphill, once or twice a week.

How long it takes. In one comparison, eight weeks of four-by-fours raised VO2 max by 5 to 10 per cent.

This week, put the hard session in your diary like a meeting, with a time and a route. You'll often feel the difference on the stairs before your watch shows it.