One fitness metric is considered the gold standard for understanding your overall health, and you may already have an idea where you stand.

Cardiorespiratory fitness, or CRF, measures how well your heart and lungs work together to deliver oxygen to your body during physical activity. In simple terms, it reflects how efficiently your body keeps you moving when you walk, exercise or perform everyday tasks.

If your CRF is lower, you may feel out of breath during activities that should not feel difficult, such as climbing stairs, carrying groceries, walking at a brisk pace or doing light exercise.

You can measure your cardiorespiratory fitness with a test called VO2 max, short for maximal oxygen consumption. VO2 max measures the maximum amount of oxygen your body can take in, circulate, and use during exercise.

“VO2 max shows how well your heart, lungs and blood vessels work together when your body is under physical stress,” says Dr. Christopher Johnson, a family and sports medicine physician at Tidelands Health. “When you exercise, you breathe in oxygen, and your body has to move that oxygen through your bloodstream to power your muscles. There is a limit to how much oxygen your body can use at one time, and that limit is what VO2 max measures.”

The most accurate way to determine VO2 max is through a clinical exercise test. During this test, you exercise on a treadmill or stationary bike while wearing a mask that measures how much oxygen you breathe in and how much carbon dioxide you breathe out.

You start at an easy pace and gradually increase your effort until you reach your maximum. A trained medical professional then reviews your oxygen use, heart rate and energy output to calculate your true VO2 max, according to Dr. Johnson.

Many people already have an estimated fitness level through devices like an Oura Ring, Apple Watch, Garmin or other wearable fitness trackers.

These devices do not directly measure oxygen intake and carbon dioxide output. Instead, they estimate VO2 max using algorithms that incorporate your heart rate during activity, age and sex, weight, fitness habits and activity patterns over time.

While this number is not exact, it can still provide a helpful snapshot of your cardiorespiratory fitness, like whether you are below average, average or above average for your age and gender.

Studies show VO2 max is one of the strongest indicators we have for predicting how long and how well we will live.

“A higher cardiorespiratory fitness and VO2 max are associated with a lower risk of early death, type 2 diabetes, cardiovascular disease and certain cancers,” Dr. Johnson says.

On the other hand, low cardiorespiratory fitness carries a significant risk. Research shows that low CRF can carry health risks similar to smoking or having uncontrolled high blood pressure, even in people who eat well and do not smoke.

There is some good news, though. 

“While you cannot change your genetics or your age, you can improve your cardiorespiratory fitness,” Dr. Johnson says. “You just have to get moving.”

Cardiorespiratory fitness improves when your heart and lungs are regularly challenged to deliver more oxygen to your body, which increases VO2 max. Over time, this makes daily activities feel easier and improves how efficiently your body uses oxygen.

Dr. Johnson recommends starting with the talk test.

• If you can sing comfortably while moving, the activity is low intensity.
• If you can talk but cannot sing, you are at a moderate intensity.
• If you can only say a few words at a time, the activity is vigorous.

Most people can improve their cardiorespiratory fitness by spending more time at a moderate intensity. A good place to start includes brisk walking for 30 minutes a day, five days a week, or swimming laps for a total of 150 minutes per week.

“The best exercise is the one you will stick with,” Dr. Johnson says.