VO2 max is the single best predictor of long-term cardiovascular health and all-cause mortality — better than BMI, blood pressure, or cholesterol. Research from the Cleveland Clinic and others consistently shows it correlates with lifespan as strongly as any biomarker we have. Here's how it works and how to improve it.
What is VO2 max?
VO2 max (maximal oxygen uptake) is the maximum rate at which your body can consume oxygen during intense exercise. It's expressed in millilitres of oxygen per kilogram of bodyweight per minute (ml/kg/min). It measures how efficiently your cardiovascular system delivers oxygen to working muscles and how effectively those muscles use it.
Elite endurance athletes typically have VO2 max values of 70–85 ml/kg/min. Recreational athletes fall in the 40–60 range. Sedentary adults often measure 30–40. Each 3.5 ml/kg/min increase in VO2 max is equivalent to one MET (metabolic equivalent) — and research shows that each additional MET is associated with a roughly 10–13% reduction in all-cause mortality.
What limits VO2 max?
VO2 max is primarily limited by:
- Cardiac output — how much blood your heart can pump per minute
- Blood oxygen-carrying capacity — haemoglobin and red blood cell count
- Mitochondrial density in muscles — how much oxygen muscles can extract from blood
- Genetics — probably 40–50% of VO2 max variation is heritable
Training can meaningfully improve the non-genetic components — primarily cardiac output and mitochondrial density. Most people are operating well below their genetic potential, meaning there's significant room to improve.
Method 1: Zone 2 training — the aerobic foundation
Zone 2 (60–70% HR max) is the primary driver of mitochondrial biogenesis — the growth of new mitochondria in muscle cells. More mitochondria means better oxygen extraction from blood, which directly supports VO2 max.
Zone 2 adaptations build slowly but durably. Think months and years, not weeks. Three to five hours of Zone 2 per week consistently produces measurable VO2 max improvement over a 12–16 week period in untrained and moderately trained individuals.
Method 2: VO2 max intervals (Zone 5)
The most efficient way to raise VO2 max directly is to train specifically at VO2 max intensity — which corresponds roughly to Zone 4–5, or 90–100% of HR max. Classic protocols:
- 4×4 Norwegian intervals: 4 repetitions of 4 minutes at 90–95% HR max, with 3-minute active recovery between. This protocol has extensive research backing and consistently produces large VO2 max improvements.
- 30/30 Billat intervals: 30 seconds at VO2 max pace, 30 seconds easy, repeated 10–16 times. More accessible but similar stimulus.
- Tabata protocol: 8×20 seconds all-out with 10 seconds rest. Extreme intensity, short duration. Effective but brutal.
One to two VO2 max interval sessions per week is generally the ceiling before recovery becomes limiting. More is not more here.
Method 3: The 80/20 combination
The training model with the most evidence behind it at the elite level — and now increasingly validated in recreational populations — is 80/20 polarised training: roughly 80% of volume in Zone 1–2 and 20% in Zone 4–5.
The logic: Zone 2 builds the aerobic base that enables high-intensity efforts. High-intensity work drives VO2 max directly. The combination produces greater VO2 max improvement than either approach alone, and recovers better than training everything at moderate intensity.
How fast can VO2 max improve?
With consistent training (5+ hours per week), untrained individuals typically see 15–20% VO2 max improvement over 12 weeks. Moderately trained people see 5–10% improvement. Elite athletes are already near their genetic ceiling and improve slowly — 1–3% per year.
The more room you have to improve, the faster it comes. Starting from low aerobic fitness, VO2 max improvements are often noticeable within 6–8 weeks.
Tracking VO2 max progress
Apple Watch estimates VO2 max based on heart rate during outdoor runs and walks. The estimate has limitations (especially at higher fitness levels), but it reliably tracks trends over time. You can see it in the Fitness app under Health Data → Heart → Cardio Fitness.
Underdog's zone points give you a complementary proxy: as your VO2 max improves, you'll be able to sustain higher zones for longer periods, earning more points per session. Track both — VO2 max for the physiological data, zone points for the competitive accountability.