VO2 max is the single best indicator of cardiorespiratory fitness.

When it comes to measuring fitness and longevity, there is one number cardiovascular experts believe rule them all: VO2 max. VO2 max — or your oxygen uptake — shows how much oxygen your body absorbs and uses while working out. V is for volume, O2 is for oxygen and max is for maximum. Many health experts consider it the single best indicator of cardiorespiratory fitness, and research has found it correlates to longevity as well.

A person’s maximal oxygen consumption, also known as VO2 max—a measurement of how well the body takes in and uses oxygen while exercising at a high intensity—is a useful statistic to gauge fitness and cardiovascular health. A higher VO2 max score means the heart and lungs are transporting more oxygen-rich blood to the muscles, increasing energy production and maximizing athletic performance.

The most accurate VO2 max tests are done in a laboratory where you are hooked up to a mask that measures oxygen consumption as you exercise. VO2 max is measured in milliliters of oxygen a minute per kilogram of body weight. The more oxygen you burn, the fitter you are.

Source:  Cleveland Clinic.

If you are on a treadmill, you are told to walk or run faster and faster until your oxygen consumption stops rising. If you go beyond this point, you will increasingly be using anaerobic energy—which doesn’t burn oxygen—and you will have trouble sustaining it for long periods.

References:

1. https://health.clevelandclinic.org/what-is-vo2-max-and-how-to-calculate-it/
2. https://www.barrons.com/articles/vo2-max-test-increase-fitness-age-128c53cd