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 run roughly 20% efficient, which means that for every 100 Watts we produce, we also produce 400 Watts of heat. That means that 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. This was seen as a protective mechanism to prevent overheating which was based on studies where participants exercised in the heat until they were exhausted, a point that correlated with a core temperature of a about 39.5 °C (103 °F).
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However, it has now become clear that an interplay of multiple factors and not just core temperature is responsible for a decrease in performance in the heat. It’s now understood that while heat affects performance, 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 you’re dehydrated, blood-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
As your brain heats up a number of changes begin to take place such as fuel depletion and changes in neurotransmitters which may adversely affect brain function.
Heat also directly and indirectly influences muscle function, with an accumulation of metabolites and an increase rate of glycogen breakdown.
Increases in ventilation and breathlessness are usually observed.
Last but certainly not least, there is increased discomfort and effects on pain tolerance, mood, and motivation, all of which can influence performance.
So what causes decreased performance in the heat?
We cant change the environment, but hydration before and during your event, as well as cooling the skin and body can increase performance in the heat.
Original Source: Asker Jeukendrup / mysportscience