What is VO2 Max Tracker: Aerobic Fitness Monitoring?

Track your VO2 max with Fitiv Pulse. Understand what VO2 max means in mL/kg/min, how it's estimated, and how to improve it with Zone 2 training.

VO2 Max Tracker: Aerobic Fitness Monitoring

VO2 max is the single most comprehensive measure of cardiovascular fitness available without laboratory equipment. It represents the ceiling of your aerobic energy system and predicts endurance performance more reliably than any other single metric. Fitiv Pulse estimates your VO2 max from workout data and tracks changes over time, giving you a concrete measure of whether your training is actually improving your aerobic fitness.

What is VO2 Max?

VO2 max (maximal oxygen uptake) is the maximum rate at which your body can consume oxygen during intense exercise, expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). It defines the upper limit of your aerobic energy production capacity.

At the cellular level, VO2 max is constrained by the cardiovascular system's ability to deliver oxygenated blood to working muscles (central limitation) and the muscles' capacity to extract and use that oxygen (peripheral limitation). Elite endurance athletes have VO2 max values in the range of 70-90+ mL/kg/min. Norwegian cross-country skier Bjorn Daehlie was measured at 96 mL/kg/min — the highest reliably recorded value. Well-trained recreational athletes typically fall in the 45-60 mL/kg/min range.

VO2 Max Reference Ranges by Age and Sex

VO2 max declines approximately 1% per year after age 25 in sedentary individuals. Regular training substantially slows this decline.

Men (mL/kg/min):

Excellent: 55+ (age 20-29), 51+ (30-39), 46+ (40-49), 42+ (50+)
Good: 48-54 (age 20-29), 44-50 (30-39), 40-45 (40-49), 35-41 (50+)
Average: 42-47 (age 20-29), 38-43 (30-39), 34-39 (40-49), 29-34 (50+)
Below average: below 42 (age 20-29), below 38 (30-39), below 34 (40-49), below 29 (50+)

Women (mL/kg/min):

Excellent: 47+ (age 20-29), 43+ (30-39), 39+ (40-49), 35+ (50+)
Good: 41-46 (age 20-29), 37-42 (30-39), 33-38 (40-49), 29-34 (50+)
Average: 35-40 (age 20-29), 31-36 (30-39), 27-32 (40-49), 23-28 (50+)
Below average: below 35 (age 20-29), below 31 (30-39), below 27 (40-49), below 23 (50+)

These ranges are based on data from the Cooper Institute and the American College of Sports Medicine.

The Relationship Between VO2 Max and Endurance Performance

VO2 max is necessary but not sufficient for elite performance. Two athletes with identical VO2 max can have significantly different race performances depending on their lactate threshold (what fraction of VO2 max they can sustain), running economy (oxygen cost of running at a given pace), and fatigue resistance.

That said, VO2 max sets the ceiling. An athlete with a VO2 max of 45 cannot out-compete one with 65, regardless of how well-trained other systems are. Improving VO2 max expands what is possible.

How Fitiv Estimates VO2 Max

Laboratory measurement of VO2 max requires a metabolic analyzer, a treadmill or cycle ergometer, and a maximal exertion protocol — none of which are practical for daily use. Fitiv uses a validated submaximal estimation approach based on the relationship between heart rate and workload.

The algorithm uses recorded heart rate during structured workouts — particularly runs and rides at known pace, distance, or power — and applies the principle that at any given workload, a lower heart rate indicates greater cardiovascular efficiency, which correlates with higher VO2 max.

Fitiv's estimate is most accurate when:

GPS pace or cycling power data is available (workload is precisely known)
The workout intensity is in the range of 70-90% of maximum HR
At least 10-15 minutes of steady-state effort is included
External conditions (heat, altitude) are approximately normal

The estimate is updated after qualifying workouts and displayed as a trend over time. As with all submaximal estimation methods, the absolute value may differ from laboratory measurement by 5-10%, but the trend over time is reliable for tracking fitness changes.

VO2 Max and Apple Watch Cardio Fitness

Apple Watch independently estimates VO2 max and labels it "Cardio Fitness" in the Health app. Fitiv incorporates this data and also derives its own estimate from workout data for comparison. When the two estimates diverge significantly, it typically indicates measurement conditions that favored one algorithm over the other. The trend from either source is more useful than a single data point.

Interpreting VO2 Max Changes Over Time

Meaningful changes in VO2 max require consistent training over weeks to months. A 1-2 mL/kg/min improvement is meaningful; day-to-day fluctuations in the estimate reflect measurement variation, not true fitness changes. Fitiv displays a 30-day smoothed trend to filter out noise.

Expect the following approximate timelines:

New athletes: 10-15% VO2 max improvement possible within 6-12 months of structured training
Intermediate athletes: 5-8% improvement per year with systematic training
Trained athletes: 1-3% improvement per year; maintaining VO2 max becomes the goal

How to Improve VO2 Max

Two distinct training approaches improve VO2 max through different mechanisms, and both are necessary for maximal development:

Zone 2 Training for VO2 Max Development

Zone 2 training (60-70% max HR) builds mitochondrial density and improves fat oxidation — the foundation of aerobic capacity. It does not directly stress the cardiovascular ceiling but builds the peripheral capacity that allows you to sustain high fractions of VO2 max for longer.

Athletes who increase Zone 2 volume substantially — 8-12 hours per week for serious endurance athletes — consistently see VO2 max improvements over 3-6 month training blocks, despite never doing what most people would call "hard" training.

High-Intensity Intervals for VO2 Max Development

Zone 5 intervals (90-100% max HR) directly stress the cardiovascular ceiling. Classic VO2 max interval protocols include:

4-6 × 4 minutes at 95-100% max HR, with 3-4 minutes recovery
6-8 × 3 minutes at maximum sustainable effort, with 2-3 minutes recovery
Norwegian 4×4 protocol (4 minutes at 90-95% max HR, 3 minutes active recovery, 4 sets)

These sessions produce rapid improvements in maximal cardiac output and stroke volume, directly raising the VO2 max ceiling. However, they are highly fatiguing and should be used sparingly — 1-2 sessions per week maximum, with adequate recovery days between.

The combination of high Zone 2 volume and targeted Zone 5 work produces greater VO2 max improvement than either approach alone.

Why VO2 Max Monitoring Matters for Athletes

Tracking VO2 max over months of training answers the fundamental question every athlete has: is what I'm doing actually working? Subjective perceptions of fitness improvement are unreliable; a concrete physiological metric with a meaningful upward trend confirms that training adaptations are occurring.

VO2 max tracking is also valuable for detecting fitness loss. Illness, overtraining, significant life stress, or a reduction in training volume will cause a measurable VO2 max decline before subjective performance degradation becomes obvious in races or workouts. Early detection allows training adjustments.

Frequently Asked Questions

Q: How accurate is Fitiv's VO2 max estimate compared to a lab test? A: Submaximal estimation methods have a typical margin of error of ±5-10% compared to laboratory graded exercise tests. For the purpose of tracking your own trend over time, this error is acceptable because it is systematic — the estimate is consistently applied using the same algorithm, so improvements in your estimated VO2 max reflect genuine fitness improvements. Do not use the app estimate to compare directly to a lab-measured value.

Q: Can I improve my VO2 max at age 50 or older? A: Yes. VO2 max is trainable at any age, though the absolute ceiling is lower with age. Studies on masters athletes show meaningful VO2 max improvements (5-10%) in response to structured training in athletes over 60. The same training principles apply — Zone 2 volume and periodic high-intensity intervals — though recovery time between hard sessions increases with age.

Q: Does weight loss improve VO2 max? A: VO2 max is expressed per kilogram of body weight, so losing fat mass while maintaining aerobic fitness mathematically increases your VO2 max value. A 70 kg athlete consuming 40 mL/kg/min would measure 49 mL/kg/min at 57 kg with unchanged fitness (40 × 70 ÷ 57 ≈ 49). Body composition changes that reduce body fat improve VO2 max without any change to cardiovascular fitness — this effect is real and adds to the performance impact of reduced body weight.

Q: How often should I check VO2 max? A: Fitiv updates the estimate automatically after qualifying workouts. There is no need to do anything special to track it. Review the trend monthly rather than after individual workouts — short-term fluctuations in the estimate reflect measurement conditions more than actual fitness changes.