About VO2Max

Apple Watch’s cardio fitness metric estimates VO₂ max: the maximum volume of O₂ (oxygen) the body can utilize during intense exercise. While the name VO₂ max sure is a mouthful, there’s a lot of science behind why it matters and how to improve it.

During clinical testing of VO2Max, you’d typically wear a respiratory max while exercising intensely (e.g., during treadmill or cycling). Apple Watch’s cardio fitness algorithm estimates VO2 max based on heart rate and GPS without a mask, to an accuracy of about 5%.

VO2 Max corrleates with longevity

Why do people care about VO2Max? While VO2Max is a measure of aerobic capacity, it’s really valued as a biomarker of longevity. Every 1 mL/kg/min increase in VO2 max reduces risk of death by 9 percent. Simply going from low cardio fitness (bottom quartile) to being below average (next quartile) is associated with a 50% reduction in mortality over a decade (2018 study in JAMA Network Open):

VO2 max has gotten more attention in recent years due to the work of Peter Attia and other longevity-focused doctors. Even before then, VO2Max was often used by sports cardiologists (such as Dr Levine, inventor of the CHOP/Levine protocol for POTS) as an important biomarker for people with chronic conditions. No matter where you are in the health spectrum, VO2Max is likely to be relevant to you.

What’s a good VO2Max for my age?

A good VO2Max varies significantly by age and sex. Anything from 27 to 40 ml/kg/min can be average. See above for a chart of VO2Max by age and sex. Or download Empirical Health to get personalized health metrics.

How to trigger VO2 max on Apple Watch

To trigger a cardio fitness (VO2Max) measurement on your Apple Watch, do a workout that meets these criteria:

• An outdoor walking/running/hiking workout
• …on flat ground (5% grade or less).
• During the workout, your heart rate needs to increase more than 30% of the way between your resting heart rate and max heart rate. For example, if your resting heart rate is 60bpm and your max heart rate is 180bpm, you’d need to reach a heart rate of 96bpm or above to get a VO2Max estimate.

Within a couple of hours after the workout, your cardio fitness will be estimated automatically. It will show up in the built-in Health app and third party Apple Watch health apps like Empirical Health.

What to do about low cardio fitness alerts from Apple Watch

If your Apple Watch gives you a low cardio fitness warning, you can increase vo2max with zone 5 training (such as Norwegian 4x4s).

Zone 5 training is intense exercise at 90-100% of your maximum heart rate. Your max heart rate is typically around 220 minus your age. For example, if you’re 40 years old, zone 5 training would be exercise at 162-180bpm. Most people can only sustain this pace for a few minutes. The fastest way to increase vo2max is interval training. The Norwegian 4x4 is one workout designed to increase vo2max.

Even if you can’t do zone 5 training, zone 2 training (lower intensity cardio) can improve VO2 max. This is particularly for those in the bottom half of their benchmarks by age and gender.

You can create a plan in Empirical Health to increase VO2Max. Empirical Health will preload Norweigan 4x4 workouts directly on your Apple Watch, in addition to letting you set customized goals for zone 2, zone 5, and strength training.

Accuracy of Apple Watch’s cardio fitness algorithm

Apple Watch estimates VO2Max without a respiratory mask by using your heart rate and GPS data. Apple ran an internal study validating their algorithm with 221 participants, and found that Apple Watch’s cardio fitness estimate was accurate to within 1.2 ml/kg/min of the true VO2Max. Apple Watch’s cardio fitness metric had a reliability coefficient of 0.86–0.89.

Here’s a chart of actual VO2Max (x axis) vs the Apple Watch’s estimate (y axis). The Apple Watch is surprisingly accurate for a wrist-based measurement:

Why VO2Max is linked to heart health

Why would VO2Max actually tell you anything about your heart? On its face, it seems like a fairly random metric. What makes it special?

One way to think about this is through the Fick equation:

Oxygen delivered = (volume of blood pumped) x heart rate x arterio-venous oxygen difference.

For most of us, the arterio-venous oxygen difference is fixed: we can’t do much about it.

And our heart rate is capped to a maximum. Max heart rate is usually around 220 - age. So we can’t change that much, either.

That leaves only one real lever to increase VO2Max: increase the volume of blood pumped with every heart beat. In other words, to make the heart muscle stronger. And indeed, studies of elite endurance athletes show their high VO2max is explained by high cardiac output from a large cardiac chamber which “relaxes quickly and fills to a large end-diastolic volume” (Levine 2007).

So in other words, VO2Max is an indirect measurement how how strong your heart is—the volume of blood pumped with every heart beat.