16 Oct 2026 4 min read science

Snippet #040: Why the Same Training Produces Such Different Results

By Thomas Mortelmans

Give hundreds of sedentary adults the same 20-week aerobic program and the spread of outcomes is striking.

In the HERITAGE Family Study, the average gain in maximal oxygen uptake (VO2max, a measure of how much oxygen the body can use at peak effort) was roughly 400 ml/min, but the range ran from near zero to more than a liter per minute.

Bouchard and colleagues found about 2.5 times more of that variation between families than within them, pointing to a heritability near 47% for VO2max trainability (Bouchard et al. 1999).

This matters because group averages routinely bury individual biology. A person who sees a modest VO2max gain from a standard endurance block is not necessarily a poor responder to training as a whole.

Strength, movement economy, and power output each carry their own adaptive profiles and genetic architecture.

Low VO2max trainability reflects one trait, not overall athletic potential. Genes set a range of possibilities, and training volume, intensity, and method all influence where within that range a person lands.

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### Health disclaimer

This post discusses endurance-training science for educational purposes. It is not medical advice, not a diagnosis, and not a substitute for clinical care. Individual response to training, sleep deprivation, and multi-day exertion varies substantially, and what applies to a research cohort or a world-class athlete may not apply to you. > Consult a qualified physician, sports medicine specialist, or registered dietitian before changing your training, fuelling, or sleep strategy if you have a cardiovascular, metabolic, psychiatric, or sleep-related condition; are recovering from injury or illness; are pregnant; are on medication that affects heart rate, hydration, glucose regulation, or sleep; or have concerns about exercise tolerance. > Ultra-endurance events impose real physiological and psychological loads. Persistent chest pain, fainting, acute confusion beyond the predictable late-race window, severe dehydration, sustained loss of coordination, or any mental-health symptoms that outlast the immediate post-event dip warrant professional care and are not signals to push through. > No outcome is guaranteed. The protocols, anchors, and case material in this post are descriptive, drawn from peer-reviewed evidence and a guest contributor's lived experience, and should be treated as inputs to an informed conversation with your own coach and clinicians, not as prescriptions.