From Fellrnr.com, Running tips
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Generally, an athlete reaches ‘volunatry exhasution’ when their core temperature reaches about 39c/102f/<ref name="Nielsen"/>. A dedicated athlete can push themselves hard enough to raise their core temperature to dangerous levels (>40c), leading to heatstroke, which can be fatal<ref name="Binkley"/>. Heatstroke can be the result of prolonged exercise in hot conditions, but it can also be the result of shorter periods of very high intensity exercise, especially in the untrained or overweight.
== references ==
<ref name="Armstron1991"> http://www.ncbi.nlm.nih.gov/pubmed/1763248 The induction and decay of heat acclimatisation in trained athletes.</ref>
<ref name="Shapiro">https://www.thieme-connect.com/ejournals/abstract/sportsmed/doi/10.1055/s-2007-971986 Acclimatization Strategies - Preparing for Exercise in the Heat</ref>
<ref name="Nielsen">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1175224/ Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment.</ref>
<ref name="Binkley">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC164365/ National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses</ref>