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Running in the heat is much harder than running in cooler temperatures, reducing performance and creating serious health risks. Moderate exercise in mild temperatures, such as 60f/15c, will reduce performance, but the perception is likely to be fatigue rather than overheating. More extreme exercise or temperatures can create the risk of heat illnesses which can be dangerous.
=Heat and Fatigue=
It is generally accepted that exercise at a constant effort will be voluntarily terminated once the core body temperature reaches about 40c/104f<ref name="Tucker-2008"/>. However, if subjects are allowed to vary their pace freely, they will preemptively reduce their exercise intensity to prevent their core temperature form reaching the critical point<ref name="Tucker-2008"/>. One mechanism for this reduction is that fewer [[Muscle|muscle fibers ]] are recruited in hot conditions, even before the core body temperature starts to rise<ref name="Tucker-2004"/>. Elevated core temperature reduces the voluntary maximum force generated, though it is unimpaired when electrical stimulation is used, indicating the central nervous system is responsible<ref name="Nybo-2008"/> . When exercising at a constant [[Rating of Perceived Exertion]], subjects steadily reduced their power output at 59f/15c, 77f/25c or 95f/35c even though their perception of their temperature comfort remained constant<ref name="Tucker2006"/>.
==Mechanisms==
Exercise in the heat burns more carbohydrate than in cool conditions<ref name="Mündel-2008"/> . However, it is not clear to me if this is because of a direct change in the use of carbohydrate, or if the hot conditions reduce [[VO2max|V̇O<sub>2</sub>max]] and therefore the exercise intensity is effectively increased. Exercise in the heat also burns more muscle [[Protein]]<ref name="Snow-1993"/><ref name="Marino-2001"/>, and the waste products from this [[Protein]] metabolism could result in mental fatigue due to a rise in [http://en.wikipedia.org/wiki/Gamma-Aminobutyric_acid GABA]<ref name="Mündel-2008"/>.