From Fellrnr.com, Running tips
Generally, an athlete reaches ‘voluntary exhaustion’ 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.
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