Your VO₂ Max Score Is One of the Most Important Numbers You Don't Know

Most people who exercise regularly can tell you their resting heart rate, their mile pace, maybe their one-rep max. Almost none of them can tell you their VO₂ Max — and it's the number that probably matters most.

VO₂ Max measures the maximum volume of oxygen your body can consume and use during intense exercise. It's expressed in milliliters of oxygen per kilogram of bodyweight per minute (ml/kg/min), and it's the closest thing exercise science has to a single number that predicts long-term health outcomes.

This is not a niche athletic metric. Research published in JAMA Network Open found that low cardiorespiratory fitness was associated with higher all-cause mortality risk than hypertension, smoking, or diabetes. [https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2707428] Your VO₂ Max is a direct measure of your cardiorespiratory fitness. It's worth knowing.

What the Test Actually Measures

A VO₂ Max test at VO2 Max SF is a graded exercise test — you exercise at increasing intensities (typically on a treadmill or bike) while wearing a mask that measures your oxygen consumption and carbon dioxide output in real time. The test continues until you reach maximal effort or can no longer maintain the required intensity.

The data produced goes beyond a single VO₂ Max number:

- Heart rate-based training zones — derived from your actual physiology, not age-based formulas

- Respiratory Exchange Ratio (RER) — the ratio of CO₂ produced to O₂ consumed, which indicates whether you're burning primarily fat or carbohydrates at different intensities

- Metabolic efficiency — how economically your body uses oxygen at submaximal intensities

- Resting Metabolic Rate (RMR) — how many calories your body burns at rest, measured rather than estimated

These markers together give a complete picture of how your cardiovascular and metabolic systems are functioning — and where the ceiling is.

What Your VO₂ Max Number Means

VO₂ Max scores are evaluated relative to age and sex. For reference, research from the American Heart Association classifies cardiorespiratory fitness into five categories, with scores above 51 ml/kg/min for men and 44 ml/kg/min for women generally considered "high." [https://www.heart.org/en/healthy-living/fitness/fitness-basics/aha-recs-for-physical-activity-in-adults]

Most recreational exercisers land in the "moderate" range. Many people who consider themselves fit are surprised to find their aerobic ceiling is lower than their training history would suggest — often because they've been training in the wrong zones.

Why Training Zones Matter

Without a VO₂ Max test, training zones are estimated. The most common method — subtracting your age from 220 to estimate max heart rate — carries an error range of plus or minus 10–12 beats per minute. For many people, this is the difference between a Zone 2 session and a Zone 4 session.

Training in the wrong zones doesn't just produce suboptimal results. It can produce counterproductive ones. Research consistently shows that most endurance athletes improve fastest with a polarized distribution — the majority of volume at genuinely low intensity, with a smaller portion at genuinely high intensity. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4621419/] Without accurate zones, neither end of the spectrum is properly calibrated.

How to Improve Your VO₂ Max

VO₂ Max is highly trainable — more so than almost any other health biomarker. The levers are:

Zone 2 training: Sustained aerobic work at low intensity — conversational pace, below the first ventilatory threshold. This builds the mitochondrial density and cardiac stroke volume that underpin a high VO₂ Max. Most people who feel "fit" are surprised by how slowly they need to run to stay in Zone 2 when zones are set correctly.

High-intensity interval training (HIIT): Short intervals at or near VO₂ Max intensity are the most direct stimulus for improving the ceiling itself. The specific protocols that work best (interval length, recovery ratio, session frequency) depend on your current score and your training history.

Consistency over intensity: VO₂ Max responds to training volume over months, not heroic single sessions. The most important variable is sustained aerobic work across weeks and years.

Retesting every 8–12 weeks allows you to see whether the stimulus is working and adjust accordingly.

Where to Get Tested in San Francisco

VO2 Max SF is located inside Custom Fit [https://www.customfitsf.com] at 1844 Market St in Hayes Valley, with additional locations in San Francisco. Testing appointments take approximately one hour and include a full review of your results.

Custom Fit members with personal training or partner training memberships have unlimited VO₂ Max testing included — meaning the retest cadence that produces the best outcomes is already built into the membership.

If you're training without knowing your cardiovascular ceiling, you're working from an incomplete map. The test takes an hour. The data lasts months.

Book your VO₂ Max test at VO2 Max SF → [https://www.vo2maxsf.com]

Learn more about training with your results at Custom Fit SF → [https://www.customfitsf.com]