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Maximal oxygen uptake improves substantially with training, but the size of the gain is highly individual and partly genetically capped — with notable "high" and "low" responders.
VO2max (maximal oxygen uptake) is the ceiling of the aerobic system — the most oxygen the body can take in and use per minute (see aerobic-system and vo2-reserve). It is the single best lab marker of cardiorespiratory fitness and a strong predictor of health and longevity.
It is trainable. Endurance and interval training reliably raise VO2max via the central adaptations (bigger stroke volume, higher cardiac output; see cardiac-adaptations) and peripheral adaptations (more mitochondria and capillaries). Typical improvements with a structured program are on the order of ~5–20%, and previously sedentary adults can gain substantially (e.g., ~40 → ~49 mL/kg/min over a year of intensive training). High-intensity interval work and 4×4-style VO2max intervals are particularly potent stimuli (see hiit, vo2max-intervals).
But trainability is individual and partly genetic. Studies of identical training doses (e.g., the HERITAGE family study) show large between-person differences in how much VO2max improves — from minimal "low responders" to large "high responders" — with a strong heritable component. There is also an individual ceiling: as someone approaches their genetically influenced maximum, further gains become progressively harder. This is a concrete example of the principle of individualization (see individuality): the same program yields different VO2max outcomes in different people, so expectations and prescriptions should be individualized.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.