Snippet #027: Your Energy Systems Run Together, Not in Sequence

The aerobic and oxygen-independent pathways of energy production operate simultaneously from the first second of exercise.

Their relative share shifts continuously with intensity and duration; it is a dial, not a switch. A landmark review by Gastin (2001) found that during all-out maximal effort, the aerobic system already supplies roughly 50% of total energy at approximately 75 seconds, considerably earlier than textbooks traditionally suggested (doi:10.2165/00007256-200131100-00003).

Even in short, high-intensity sprints, oxidative metabolism is contributing substantially.

The common framing of aerobic athlete versus oxygen-independent athlete treats the two systems as mutually exclusive, but the data show they are both active throughout exercise. What changes is the ratio. A sprinter's aerobic capacity shapes recovery between efforts.

An endurance athlete's ability to briefly exceed steady-state output shapes race tactics.

Precise estimates of this split vary with measurement method and participant fitness level. The 75-second crossover is a central estimate across the mixed-fitness populations studied in maximal-effort protocols.

Gradescale: Your physiology, decoded. Join the waitlist now.

Introducing Gradescale: An Endurance Analytics Platform That Shows Its Work

A contribution to the science-curious end of the endurance community Over the past few months, this corner of the endurance world has been working through one half of a workflow. The half this newsletter explores is the science. Physiology, dose-response curves, how stimuli actually drive adaptation, the difference between what

The Scientist’s NotebookThomas Mortelmans

If you have 60 seconds, I would value your anonymous feedback. You can share it here.

---

### 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.