Why is performance in the heat decreased?

Mechanisms of impaired performance in the heat

It is generally accepted that aerobic exercise capacity in hot conditions is reduced whilst sprint performance may even be improved (see previous blog). The impact of environmental temperature and humidity on endurance performance can be significant. Marathons are slower when the temperatures rise above 11oC and many Ironman competitions are held in very hot conditions. This article will discuss why performance is impaired in the heat.

Heat production is directly proportional to exercise intensity, so extremely strenuous exercise, even in a cool environment, can cause a substantial rise in body temperature. Humans are roughly 20% efficient, which means that for every 100 Watts we produce, we also produce 400 Watts of heat. Of course athletes who produce 400W would produce a whopping 1600Watts of heat! The body has various ways to remove this heat and sweating is often the most important one. Sweating will allow an athlete to remove heat but it may also result in dehydration, which eventually makes it more difficult to regulate body temperature. When the environment is hot and humid is becomes more difficult to remove this heat through conduction and convection and we have to rely solely on sweating.

Large increases in body temperature during exercise are unlikely to occur in individuals who run at a slower pace (e.g., those who run a marathon in 4 to 6 hours) but are common in the faster, highly motivated athletes (who are able to produce more power and thus more heat).

For a while it was thought that when body temperature rises to about 39.5 °C (103 °F), central fatigue (i.e., fatigue in the brain rather than in the working muscles) would develop (1). This was seen as a protective mechanism to prevent overheating. It was based on studies, where subject exercised in the heat till they were exhausted and always seemed to stop when core temperature reached about 39.5 °C (103 °F).

However, it has now become clear that an interplay of multiple factors and not just core temperature is responsible for performance decrements in the heat (Nybo et al. 2014) (Figure X).

It is now understood that heat per se affects performance but that hypo hydration makes things worse. If the body heats up and the body becomes dehydrated to a significant degree all physiological functions are likely to be compromised.


When becoming dehydrated plasma volume may be reduced, whilst blood vessels expand. This makes it harder to maintain blood pressure and blood flow, whilst heart rate is increased. If the cardiovascular system is compromised this may affect oxygen delivery and metabolite removal.

Central Nervous system

The brain will heat up and there are a number of changes in the brain like fuel depletion, changes in neurotransmitters , accumulation of ammonia, cytokines etc that may alter brain function.