Training raises VO2 max, but how much varies widely
Medium confidence
Training raises VO2 max (the most oxygen the body can use), but the size of the gain differs a lot from person to person. On one identical 20-week program, the average gain was about 400 ml/min, yet some people gained almost nothing and others gained over 1 litre per minute, and the pattern ran in families. People who start less fit, and programs that run longer, tend to see bigger gains. Most people who seem not to respond do improve once the training dose goes up.
Why it works
Gains come as the heart pumps more blood per beat, total blood volume rises, and the muscles get better at using oxygen. Genetics influence both the ceiling and how fast the body adjusts.
What it means in practice
Don't promise a fixed number. Treat training as setting up the conditions for the body to adjust, not a guarantee of a specific gain. When a runner isn't seeing the progress they expected, don't assume they're doing something wrong: some of the variation is biological, and a bigger or longer training dose often unlocks progress.
The evidence
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Joyner and Lundby note that some individuals show little or no VO2max increase on training programs consistent with public health guidelines, but that results from studies using more intense programs challenge the concept that some humans have limited trainability. They argue that trainability is context-dependent, so an apparent 'non-responder' may reflect the training dose as much as biology, and propose a new set of twin studies to separate true genetic limits from inadequate training.
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VO2max rose by about 400 ml/min on average, but responses varied widely: some people gained little or nothing while others gained more than 1.0 L/min. There was 2.5 times more variance in the response between families than within them, and the best-fitting models gave a maximal heritability of 47% for the VO2max response.
n=481
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Both endurance training (+4.9 mL/kg/min) and HIT (+5.5 mL/kg/min) produced large VO2max gains vs no-exercise controls, with HIT possibly slightly better than endurance training (+1.2). Gains were larger in people with lower baseline fitness (small extra effect for endurance training, moderate for HIT), in longer interventions, and, for endurance training, in younger participants.
n=723
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The share of people who did not improve cardiorespiratory fitness fell steadily as weekly training rose from 60 to 300 minutes. After the second 6-week block with 120 more minutes a week, non-response disappeared in every participant. The size of the fitness gain was mainly explained by increases in haemoglobin mass.
n=78
Why we call confidence medium
The HERITAGE Family Study (Bouchard 1999) trained 481 sedentary adults on the same 20-week program: VO2 max rose about 400 ml/min on average, but some gained little or nothing while others gained more than 1 L/min, and the response clustered in families (heritability up to 47%). Milanović's 2015 meta-analysis of 28 controlled trials found larger gains in people with lower starting fitness and in longer programs. Montero and Lundby (2017) showed that apparent non-response fades when the training dose rises, and Joyner's 2018 review makes the same point: some people gain little on guideline-level programs, but harder programs bring gains for most.
Where it applies
Adult runners at every training level. Beginners and detrained adults see the largest gains.
Does not apply to: children, where growth-related changes confound training adaptation; elite athletes near their physiological ceiling.
Last reviewed Oct 14, 2026. See how we score.