07/02/2026
🧠Fatigue in Endurance Exercise: A New Perspective by Timothy Noakes
Professor Timothy Noakes has recently updated his thinking on what really causes fatigue during prolonged endurance exercise — and it challenges one of the most deeply rooted beliefs in sports science.
🔬 The key idea:
Fatigue is not primarily caused by muscle glycogen depletion.
Instead, Noakes argues that exercise-induced hypoglycemia (low blood glucose) is the main trigger of fatigue.
How does this work?
During long or intense exercise, blood glucose levels can fall.
The brain is highly dependent on glucose and closely monitors its availability.
When glucose drops too low, the brain activates a protective mechanism:
👉 it reduces motor drive and perceived capacity to continue exercising.
This results in fatigue as a safety signal, not as a true failure of the muscles.
Why this matters:
Muscles rarely “run out of energy” in a catastrophic way.
ATP levels in muscle cells are usually preserved.
Athletes often stop exercising at the same time blood glucose reaches critically low levels.
Fatigue, therefore, is centrally regulated by the brain, not locally caused by the muscles.
Connection to the Central Governor Theory
This updated model fits perfectly with Noakes’ long-standing Central Governor Theory, which proposes that:
The brain regulates exercise performance to protect the body from harm — in this case, from dangerously low blood glucose that could impair brain function.
Practical implications for athletes:
Performance is not just about “full glycogen stores”.
Maintaining stable blood glucose during exercise is crucial, especially in long events.
This does not mean constant high-carb intake for everyone:
Fat-adapted athletes may need very small amounts of carbohydrate, or simply better metabolic control.
The goal is glucose stability, not sugar overload.
In short:
👉 Fatigue is not a muscle failure.
👉 It is a brain-driven protective response to falling blood glucose.
This perspective changes how we think about fueling, pacing, and endurance performance.
