VO2 max, or maximal oxygen uptake, is the most oxygen your body can take in and use each minute when you're exercising as hard as you can. It's expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min) and caps how much energy your aerobic system can produce. It's the most widely used lab measure of cardiovascular fitness and endurance potential.
What Determines VO2 Max
The "V" stands for volume, "O2" for oxygen, and "max" for maximum. The number reflects how well your heart, lungs, and muscles work together when you push them to the limit. At VO2 max, your heart is pumping at or near its maximum output, you're breathing close to as hard as you can, and your muscles are using oxygen at their physiological limit.
Four factors set that limit. Cardiac output is how much oxygenated blood your heart pumps per minute (heart rate × stroke volume). Oxygen-carrying capacity depends on hemoglobin concentration and red blood cell volume. Mitochondrial density and oxidative enzymes determine how well muscles extract and use oxygen from the blood, and capillary density affects how quickly that oxygen reaches working fibers.
In most athletes, the main limiter is cardiac output, specifically stroke volume (the volume of blood ejected per heartbeat). Elite endurance athletes have significantly larger left ventricles and higher stroke volumes than sedentary people, a structural adaptation built over years of training.
Typical ranges for comparison:
- Sedentary adult men, 30–40 years: 35–45 mL/kg/min
- Trained male recreational athletes: 50–60 mL/kg/min
- Elite male endurance athletes: 70–85 mL/kg/min
- Professional cyclists and cross-country skiers: 80–97 mL/kg/min
- Sedentary adult women, 30–40 years: 28–38 mL/kg/min
- Elite female endurance athletes: 60–75 mL/kg/min
VO2 max falls about 1% per year with age in sedentary people and about 0.5% per year in people who stay aerobically active. A 60-year-old who has trained consistently through adulthood can maintain a higher VO2 max than an untrained 40-year-old.
VO2 max is also a strong independent predictor of all-cause mortality and cardiovascular disease risk. In a study of over 120,000 patients, the mortality risk tied to low cardiorespiratory fitness was as large as or larger than the risk tied to smoking or diabetes (Mandsager et al., 2018). Each 1 MET increase in VO2 max (approximately 3.5 mL/kg/min) is associated with roughly 13% lower all-cause mortality and 15% fewer cardiovascular events (Kodama et al., 2009). If your Apple Watch rates your Cardio Fitness as Low, see what that level means and how to raise it.
How VO2 Max Relates to Training
VO2 max intervals, sustained efforts at 90–95% of maximum heart rate for 3–8 minutes, are the most effective way of directly improving VO2 max. One well-studied version is the 4x4 protocol: four 4-minute intervals at 90–95% max HR with 3 minutes of recovery, backed by randomized trial evidence (Helgerud et al., 2007).
A higher VO2 max doesn't automatically make you faster on race day. Fractional utilization (the percentage of VO2 max you can sustain for a given event) and running economy or cycling efficiency also decide the result. Picture two runners at a 10K with the same VO2 max: the one who can hold a bigger share of it, or who runs more economically, has the edge. VO2 max still sets the ceiling, so between two athletes with equal fractional utilization, the one with the higher VO2 max will be faster.
Untrained people can often raise VO2 max by roughly 10–20% over a few months of structured training, while well-trained athletes see much smaller gains. The upper limit is largely genetic, but most people have plenty of room to improve before they get near it.
How FITIV Uses VO2 Max
FITIV Pulse shows your VO2 max, which Apple calls Cardio Fitness, in your post-workout summary. If your Apple Watch recorded a Cardio Fitness reading at the end of the workout (it usually does after Outdoor Walk, Outdoor Run, and Hiking workouts), FITIV uses that value. Otherwise, including workouts with a heart rate monitor, FITIV estimates it from your peak heart rate during the workout and your resting heart rate, so harder workouts score higher. FITIV classifies the result as Poor, Active, Fit, or Elite for your age and sex; our VO2 max guide has the full ranges. To see your trend, compare similar workouts over weeks and months.
FITIV also tracks your time in each heart rate zone, so you can see how much high-intensity work, the kind that drives VO2 max gains, is in your training.
Frequently Asked Questions
What is a good VO2 max?
"Good" is relative to age and sex. In FITIV's ranges, a 35-year-old man is Fit from 43 mL/kg/min and Elite from 52. A 35-year-old woman is Fit from 30 and Elite from 38. Top endurance athletes have VO2 max values of 70–90+ mL/kg/min.
Can VO2 max be improved at any age?
Yes. While VO2 max does decline with age, it responds to training at every age. Studies in adults aged 60–80 demonstrate 10–20% VO2 max improvements following structured aerobic training programs. The adaptations are similar to those seen in younger individuals (increased stroke volume, improved muscle oxidative capacity), though the rate of adaptation may be slower and the absolute ceiling lower.
How accurate is FITIV's VO2 max estimate compared to a lab test?
Any VO2 max estimate from a wearable is less precise than a lab test. Apple Watch usually records Cardio Fitness during Outdoor Walk, Outdoor Run, and Hiking workouts, and FITIV shows that reading when there is one. Otherwise, FITIV estimates it from your peak heart rate during the workout and your resting heart rate, so harder workouts score higher. Compare similar workouts over weeks and months rather than single values.
