When Heat Takes Control: My Two Close Calls and the Science of Fatigue

The Speed Read

Heat has delivered some of my toughest racing lessons: Beijing 2008's gradual fade while leading the Olympic triathlon (to finish 12th), and London 2010's sudden blackout from exertional heat stroke (>41 °C core) with 500 m left. These weren't failures in fitness or motivation – they were thermal overload shutdowns. Your body loses heat via radiation, conduction, convection, and evaporation (the MVP, removing ~2.4 MJ/litre), but high humidity blocks evaporation, trapping heat. Traditional fatigue models, peripheral (muscle limits), central governor (brain protection), and psychobiological (effort vs. motivation), don't fully explain extreme heat's instant override. When core temperature exceeds critical thresholds, emergency protection forces cessation, no negotiation.

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Heat has taught me some of my hardest lessons in racing.

First, Beijing 2008. I'd spent months preparing for a hot race, training in hot environments and, at the time, I had even been studying environmental physiology — testing pre-cooling vests, ice slushies, and heat chamber sessions. I felt prepared for the Olympic triathlon and was excited to fulfil my dream of competing at the Olympic Games. With just 3 km left on the run, I was leading.

Then it hit.

Legs like lead. Vision narrowing. Power gone. I staggered to the finish in 12th place — conscious, but devastated. There was nothing I could do but fade gradually as heat slowly eroded control.

Two years later, London — my home World Series race. It was warm and humid, but not extreme. The crowd was huge. I’d never lost a World Series race, and the Olympics were only two years away. The pressure was high.

Halfway through the run, I was struggling, but I locked onto the back of my rival, Javier Gómez. I ignored every warning: the pain, the heavy breathing, the burning legs, the rising effort — even the strange cool tingling sensation on my skin. With 500 m to go, it was just us. The plan was simple: sit on his shoulder, then sprint.

Then everything went black.

I woke up in hospital. Ice packs. IVs. Doctors. My core temperature had exceeded 41 °C. The diagnosis: exertional heat stroke. Nine athletes passed me in those final metres. I remembered none of it.

These weren’t failures of fitness or motivation. They were physiological shutdowns from heat overload. Beijing was gradual. London was near-instant. Both permanently changed how I understand fatigue in endurance sport.

How Your Body Loses Heat — and Why Sweating Is the MVP

During exercise, the body sheds heat through four mechanisms:

• Radiation: Heat radiates to cooler air — largely ineffective when ambient temperature exceeds skin temperature (~35 °C).
• Conduction: Heat transfer to cooler objects — minimal during running or cycling.
• Convection: Air movement carries heat away — limited in still or hot conditions.
• Evaporation: Sweat vaporises, removing ~2.4 MJ of heat per litre — accounting for up to 80–90% of heat loss during exercise in hot environments (Sawka et al., 2011).

Evaporation is the dominant mechanism because it is the most efficient. But it depends on humidity. When humidity is high and sweat drips off the body instead of evaporating, cooling efficiency collapses. This “ineffective sweating” leads to rapid heat storage, accelerating fatigue and pushing core temperature upward.

Heat Buildup and the Limits of Fatigue Theory

All exercise produces heat. Only ~20–25% of energy expenditure is converted into mechanical work; the remaining ~75–80% is released as heat. When heat production exceeds heat loss, core temperature rises.

In trained athletes, core temperatures of 39–40 °C can be reached within 45–60 minutes of intense exercise in the heat — even when hydration is adequate (Nybo et al., 2014).

By the time core temperature reaches ~38.5–39.5 °C:

• Power output and maximal force decline substantially, often in the range of ~10–15% per °C under high-intensity conditions (Nybo & Nielsen, 2001).
• Muscle glycogen utilisation accelerates (Febbraio, 2001).
• Blood flow is increasingly diverted to the skin, raising cardiovascular strain and heart rate while limiting muscle oxygen delivery (González-Alonso et al., 2008).
• Perceived effort rises sharply (Nybo, 2008).

Above this range, performance declines rapidly. In some cases — like London — the system fails altogether.

Why does this happen? Several dominant fatigue models offer partial explanations, but I don’t think any of them fully capture what occurs under extreme thermal strain.

Peripheral Fatigue (Classical Model)

The traditional view of fatigue focuses on the muscle: glycogen depletion, metabolite accumulation, and impaired excitation–contraction coupling. This model explains gradual performance decline, but it cannot explain the sudden loss of consciousness. In London, failure was abrupt, not the result of progressive peripheral limitation.

Central Governor Model

The Central Governor Model proposes that the brain subconsciously regulates motor output to prevent catastrophic physiological failure by reducing effort before homeostasis is threatened (Noakes, 2012).

Heat is recognised as a regulated stressor. However, exertional heat stroke reveals a key limitation: regulation does not always occur in time. Instead of a smooth reduction in output, control can be lost entirely once core temperature exceeds a critical threshold. This suggests that beyond a certain thermal load, anticipatory regulation gives way to emergency shutdown.

Psychobiological Model

The psychobiological model frames fatigue as a decision-making process governed by perceived effort relative to motivation (Marcora et al., 2009). Exercise continues as long as perceived effort remains tolerable.

In London, my motivation was at its peak. A home race, a potential win on the Olympic course. Yet performance did not merely decline; consciousness was lost. This highlights a hard limit: perception and motivation cannot override physiological failure when thermal strain becomes extreme.

Thermal Overload: A Non-Negotiable Limit

Extreme hyperthermia is a condition in which fatigue is no longer regulated by pacing, perception, or motivation. When evaporative cooling cannot keep pace with heat production and core temperature exceeds ~40–41 °C, the body initiates an emergency protective response.

At this point:

• Cerebral function becomes impaired
• Motor control deteriorates
• Consciousness may be lost

This is not fatigue in the conventional sense. It is a forced cessation. There is no override.

Heat, therefore, is not just another contributor to fatigue. At high enough levels, it becomes a hard physiological constraint that sits outside traditional fatigue models.

The Bigger Picture

Heat accelerates fatigue, but extreme heat enforces shutdown.

Since those races, I’ve fundamentally changed how I approach preparation, pacing, and cooling strategies. In the next post, I’ll break down the science of heat acclimation — how the body adapts, and how those adaptations can be trained deliberately.

References

Sawka MN, et al. (2011). Physiological Responses to Exercise and Fluid Replacement. Comprehensive Physiology.https://onlinelibrary.wiley.com/doi/full/10.1002/cphy.c100082

Nybo L, et al. (2014). Performance in the Heat—Physiological Factors of Importance for Hyperthermia-Induced Fatigue. Comprehensive Physiology. https://onlinelibrary.wiley.com/doi/full/10.1002/cphy.c130015

Nybo L & Nielsen B. (2001). Hyperthermia and central fatigue during prolonged exercise in humans. Journal of Applied Physiology. https://journals.physiology.org/doi/full/10.1152/jappl.2001.91.3.1055

Nybo L. (2008). Hyperthermia and fatigue. Journal of Applied Physiology.https://journals.physiology.org/doi/full/10.1152/japplphysiol.00910.2007

Febbraio MA. (2001). Alterations in Energy Metabolism During Exercise and Heat Stress. Sports Medicine.https://link.springer.com/article/10.2165/00007256-200131010-00003

González-Alonso J, et al. (2008). Reductions in systemic and skeletal muscle blood flow and oxygen delivery limit maximal aerobic capacity in humans. Journal of Physiology.https://physoc.onlinelibrary.wiley.com/doi/10.1113/jphysiol.2007.142158

Noakes TD. (2012). Fatigue is a brain-derived emotion that regulates the exercise behavior to ensure the protection of whole body homeostasis. Frontiers in Physiology. https://www.frontiersin.org/articles/10.3389/fphys.2012.00082/full

Marcora SM, et al. (2009). Mental fatigue impairs physical performance in humans. Journal of Applied Physiology.https://journals.physiology.org/doi/full/10.1152/japplphysiol.91324.2008

About the Author

Alistair Brownlee is a two-time Olympic gold medallist, Ironman Champion, and co-founder of Truefuels. He is driven by a belief in science-backed training, clear structure, and removing friction from performance.