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From Fellrnr.com, Running tips
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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 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[[Protein]]<ref name="Snow-1993"/><ref name="Marino-2001"/>, and the waste products from this protein [[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"/>.
=Hyperthermia - Heat can kill=
Running in the heat can kill, as the body's protective mechanisms that normally cause us to slow down can be ineffective. There are a number of risk factors for serious heat injury
* Dizziness or confusion. This is a serious symptom that suggests either extremely low blood sugar or heatstroke.
* Flushed/Hot Skin. I've found that an early warning sign is the feeling that my skin is burning, especially my face.
* Panting. Another symptom I've found of overheating is that my breathing [[Breathing]] becomes labored beyond what is reasonable for the exercise intensity. * Core Temperature. The only sure test is to check your core temperature using something like an in-ear thermometer. Using a mouth thermometer may not be accurate if you've been breathing [[Breathing]] hard. If you have any doubts, stop and check your temperature. Avoid high intensity interval training [[High Intensity Interval Training]] in the heat; the intense work can spike your core temperature too high too quickly for you to recover. It's possible that a runner that suffers heat stroke may exhibit none, or only one of these warning signs.
=Tips for Running in the Heat=
* Plan ahead; carry more drink with you than you expect to need.
* For speed work, prefer shorter intervals and be very cautious.
* Take walking breaks [[Walking Breaks]] to cool off if you are having problems
* Watch out for the warning signs listed above. I find nausea is the first sign I have of overheating and headache is my first sign of dehydration.
* Carry a cell phone. If you get it wrong, you can call for help. (This is true in many situations.)
