VO2 Max — What the Number Predicts, and What It Doesn't

A reader in his late forties got a smartwatch in March and has been tracking his VO2 max obsessively for two months. He started at 38 ml/kg/min ("below average"). After eight weeks of zone 2 training he's at 41 ("average"). He wants to know how to get to 50 in another two months. The honest answer is: probably he can't, but probably the number isn't even the right target.

VO2 max is a real measurement of a real physiological capacity. It is also one of the most over-interpreted numbers in consumer fitness. Separating the signal from the noise requires understanding what the test actually measures.

What VO2 Max Is

VO2 max is the maximum rate at which the body can take in, transport, and use oxygen during peak exercise. It is measured in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). A trained marathon runner might measure 65–80. A sedentary office worker in their 40s might measure 30. A world-class endurance athlete might exceed 85.

The measurement is the integral of multiple capacities: cardiac output, lung diffusion, blood oxygen carrying capacity, muscle mitochondrial density, and the ability of muscle to extract oxygen from blood. Improvements come from training adaptations across all of these systems — not a single one.

What the Watch Is Estimating

Lab measurement of VO2 max requires a metabolic cart, a mask, and a graded exercise protocol pushed to exhaustion. Consumer wearables don't have access to your gas exchange. They estimate VO2 max from a regression on your heart rate response to running pace or cycling power output.

The estimate is reasonably good at the population level. Comparing a wearable's estimate to a lab measurement across thousands of users, the average error is around ±10–15%. The wearable will get the rough range right.

The estimate is unreliable at the individual level. Your wearable estimate can change by 3–5 points based on whether you ran in heat, whether you were fatigued, whether you wore the watch tight enough, whether the GPS was accurate, whether the algorithm decided your last hard interval was actually hard. Trending the estimate week to week catches mostly noise.

The estimate is most accurate for trained runners. The algorithms were validated mostly on runners. If you cycle, swim, lift, or do mixed training, the watch's estimate is closer to a guess than a measurement.

What the Number Actually Predicts

This is where the science is strong and the watch is mostly correct in direction.

All-cause mortality. Population studies consistently find VO2 max to be among the strongest predictors of mortality across all causes, dose-response with no apparent ceiling — higher VO2 max correlates with lower mortality risk even at the top of the distribution. The largest studies report that improving from "low" to "below average" cardiorespiratory fitness reduces mortality risk by roughly 50% — a larger effect than essentially any pharmaceutical intervention.

Cardiovascular event risk. VO2 max is one of the most reliable predictors of cardiovascular events in middle and older age, after standard risk factors. A 60-year-old with a VO2 max of 38 has dramatically lower 10-year cardiovascular risk than a 60-year-old with a VO2 max of 22.

Healthspan — years of functional independence. Above a critical threshold of roughly 18 ml/kg/min in older adults, basic activities of daily living are easy. Below it, climbing stairs becomes labored. The decline of VO2 max with age (roughly 10% per decade after 30) explains much of why physically inactive 75-year-olds struggle with tasks that were trivial at 50.

What the Number Doesn't Predict

The marketing claims that get oversold.

Athletic performance, in most cases. VO2 max is necessary but not sufficient. Two athletes with identical VO2 max can have very different race times. Running economy (oxygen cost per kilometer), lactate threshold (fraction of VO2 max sustainable), and race-day strategy all matter as much or more for finishing time.

Strength, power, or speed. VO2 max is a cardiovascular metric. Sprint athletes often have unremarkable VO2 max. Strength athletes often have low VO2 max. The number does not predict performance in any non-endurance domain.

Day-to-day energy or productivity. Cardiovascular fitness affects energy in long-horizon ways, but a single point of VO2 max change is not going to be felt as "more energy." That kind of subjective change comes from sleep, training load, and stress factors that the metric doesn't capture.

What You Can Actually Change

Realistic VO2 max gains for someone training appropriately:

Untrained adult: 15–25% improvement in 6 months is common. Going from 30 to 36 ml/kg/min is very achievable with consistent training.

Moderately trained adult: 5–10% improvement in 6 months requires focused effort. The diminishing returns kick in fast.

Well-trained adult: 2–5% improvement over a full year of structured periodized training. The gap between a 65 and a 70 is years of work for most people, not weeks.

The training that drives VO2 max isn't mysterious: a roughly 80/20 split between low-intensity aerobic work (zone 2) and high-intensity intervals (VO2 max work — 3–5 minute hard efforts at 90%+ effort). Volume matters. Consistency matters more. Most people who fail to improve do so because they train at moderate intensity all the time — too hard to recover from, not hard enough to drive adaptation.

What This Means for Most Readers

For a non-athlete adult past 40, the goal is rarely the absolute number. It's the trajectory.

Above 40 ml/kg/min for men, above 35 for women in middle age — you are already in a fitness range associated with substantially lower mortality risk. Marginal improvements past this point yield diminishing returns relative to the time investment.

Below 30 ml/kg/min in middle age — this is the range where the mortality and healthspan signal is strongest. Modest training delivers outsized longevity returns. This is the population for whom the metric truly matters.

The trend over years matters more than the value this month. If your number is stable through your 40s and 50s while your peers' numbers decline, you are winning the part of this race that matters.

VO2 max is one of the most predictive single numbers consumer fitness offers. It's also one of the easiest to misinterpret. The watch's daily estimate is mostly noise; the long-run trend is mostly signal. Train consistently, stop checking weekly, and the number will move where it can move. The longevity benefit is in the training itself, not in the watching of the number.