VO₂max: how fit is your cardiovascular system?

How much oxygen your body can use

VO₂max is the largest amount of oxygen your body can take in from the air you breathe and use in its metabolism. It depends on several organ systems at once, from breathing to muscle, which is why it is considered a good overall measure of endurance fitness. It is usually given per kilogram of body weight, in millilitres of oxygen per kilogram per minute (ml/kg/min)¹.

It is measured precisely in a lab: on a treadmill or bike the load rises in steps while a mask records how much oxygen you breathe in and out. Much more often it is extrapolated from a lighter effort, because that is less demanding and less tiring¹.

What is normal for your age?

VO₂max falls with age, and men are higher than women on average. That is what the US registry FRIEND shows, with directly measured values of 4,611 men and 3,172 women without known cardiovascular disease², and so does its update with lab values from 34 institutions: the typical values fell with age in men and in women, on average by about 4 ml/kg/min per decade (treadmill tests)³.

The steps say where you stand among people of the same age in this study: “Good” means higher than three in four, “Very good” higher than nine in ten. Other tables draw the limits differently, and a watch only estimates VO₂max roughly. So take the step as a direction, not an exact verdict. “Room to grow” is not a diagnosis but a starting point.

What affects your VO₂max

• ↑raises: regular endurance training, over weeks⁴
• ↑raises: high-intensity interval training⁴
• ↑raises: good genetic predisposition¹
• ↓lowers: older age^{2, 3}
• ↓lowers: weeks of little activity¹

A review of 28 controlled studies in healthy adults aged 18 to 45 found that endurance training raised VO₂max by about 5 ml/kg/min on average compared with people who did not train. High-intensity interval training reached about 5.5 ml/kg/min, slightly more⁴. With lower baseline fitness the gain was somewhat larger.

According to a literature overview in Apple’s white paper, roughly 50 to 70 per cent of the differences between people are due to genes. With inactivity, declines of up to 27 per cent have been described after just two to three weeks¹. So even a well-trained cardiovascular system needs regular stimulus.

How your watch estimates VO₂max

The Apple Watch extrapolates VO₂max from how your heart rate responds to movement. You get a value after walking, running or hiking outdoors on fairly flat ground (less than 5 per cent incline), with a good GPS and pulse signal and enough effort. The first such workout does not yet produce a value. The estimate ranges from 14 to 60 ml/kg/min¹.

It is not precise. In a study of 30 healthy adults the watch was on average about 6 ml/kg/min below the lab reference, the mean absolute error was about 7 ml/kg/min (around 13 per cent), and the difference varied widely between individuals⁵. A follow-up study of 40 adults with the Apple Watch Series 10 found a similar picture⁶.

Why VO₂max matters for health

In many studies, higher endurance fitness goes along with a lower risk of death. In a meta-analysis of 33 observational studies of healthy men and women, the risk of dying during follow-up was on average 13 per cent lower per additional MET of fitness. One MET is about 3.5 ml/kg/min^{1, 7}.

A large analysis of more than 122,000 people who had a treadmill stress test found the same link: people who were less fit than others of their age and sex died more often during follow-up⁸. These are associations, not proof that a higher VO₂max alone lowers the risk.

What Salvor does with it

Salvor takes VO₂max from your health data and shows it with its trend among the body values. It is measured only now and then, so the line carries the latest measurement forward until a new one arrives.

If your profile has age and sex and you are between 20 and 79, Salvor puts the value into one of the five steps with the table above and shows how much is missing to the next. Without those details Salvor shows the value without a rating. VO₂max does not feed into your daily form.

When to see a doctor

A low or falling watch estimate is not a diagnosis. If you have symptoms such as shortness of breath on light exertion, pain or pressure in the chest, dizziness or a racing heart, talk to a doctor, whatever a watch shows. In an emergency, call 112.