Most running watches now show a VO2 max figure. It is a helpful nudge, but it is a prediction from your pace and heart rate, not a measurement, and it says nothing about the two other numbers that decide how fast you can actually run a 10 km or a marathon. VO2 max testing for runners measures all three directly, in a laboratory, on a treadmill.

Decades of exercise physiology keep pointing to the same short list. In a widely cited review, Bassett and Howley described distance running performance as the product of three factors: your VO2 max, your running economy, and the fraction of your VO2 max you can hold for the length of your race ( Bassett and Howley, 2000).

• VO2 max the size of your aerobic engine: the most oxygen your body can take in and use each minute, per kilogram of body weight.
• Running economy how much oxygen it costs you to hold a given pace. Two runners with the same VO2 max can run different times because one is more economical.
• Fractional utilisation in practice your ventilatory or lactate threshold. This is how much of the engine you can use before fatigue climbs steeply.

A watch estimates the first and infers the rest. A test measures the engine and finds the thresholds where your training should be anchored.

At our Leichhardt clinic the test runs on a treadmill. You warm up, then speed and gradient rise in stages while you wear a mask that analyses every breath. It usually takes 8 to 15 minutes to reach the point where you cannot hold the pace. From the breath by breath data we read your VO2 max, your first ventilatory threshold (VT1) and second ventilatory threshold (VT2), your heart rate and pace at each, and the point where your body shifts from burning mostly fat to mostly carbohydrate.

Dr Samra walks through the physiology behind this in our education series video on VO2 max, mitochondria and aerobic fitness.

The most practical output is a set of training zones anchored to your measured thresholds rather than to a formula. The 220 minus age estimate of maximum heart rate can be out by 10 beats per minute or more for an individual, which is enough to turn an easy run into a moderate one and blunt the training effect.

With measured thresholds, your easy runs sit clearly below VT1, where aerobic development and fat use are highest. Your tempo and threshold sessions sit around VT2. Your interval sessions sit above it. The zones are yours, not a table's. This is also why individualised running coaching built on a real test tends to produce cleaner progress than training to generic percentages.

Within limits, yes. A systematic review and meta analysis by Milanovic and colleagues pooled controlled trials and found that both interval training and continuous endurance training improve VO2 max in adults, with higher intensity work tending to produce the larger gains ( Milanovic and colleagues, 2015). Individual response varies, which is exactly why a retest after a training block is informative: it shows whether the work you did moved the number, or whether the plan needs changing.

A strong aerobic engine sitting on top of low bone density, low energy availability or negligible strength work is a stress injury waiting to happen. Many runners benefit from reading the aerobic test alongside a DEXA body composition scan and a conversation about bone health, particularly when training volume is high or fuelling has been inconsistent. Strength and mobility work protects the running you want to keep doing.

Cardiorespiratory fitness is also one of the strongest measurable predictors of long term health. In a cohort of more than 122,000 adults, Mandsager and colleagues found that higher fitness was associated with lower mortality, with no point at which additional fitness stopped helping ( Mandsager and colleagues, 2018). We have written separately on what cardiorespiratory fitness reveals about long term health.

• Bassett DR Jr, Howley ET Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine and Science in Sports and Exercise. 2000;32(1):70 to 84. https://doi.org/10.1097/00005768-200001000-00012
• Milanovic Z, Sporis G, Weston M Effectiveness of high intensity interval training and continuous endurance training for VO2max improvements: a systematic review and meta analysis of controlled trials. Sports Medicine. 2015;45(10):1469 to 1481. https://doi.org/10.1007/s40279-015-0365-0
• Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W Association of cardiorespiratory fitness with long term mortality among adults undergoing exercise treadmill testing. JAMA Network Open. 2018;1(6):e183605. https://doi.org/10.1001/jamanetworkopen.2018.3605