Muscle contraction is only about 20-25% efficient; the other 75-80% becomes heat.A hard session produces over 1000W of heat — like running a space heater inside your body. This heat must be dissipated, or core temperature rises 1°C every 5-10 minutes, quickly becoming fatal.
2 · Two heat-loss routes
The body sheds heat through skin blood flow and sweat evaporation, and in hot environments evaporation does most of the work.Two routes: ① Skin blood flow — blood carries heat to the surface for radiation/convection; ② Sweating evaporation — 1 liter of sweat removes ~580 kcal. In hot environments, evaporation dominates, accounting for over 80% of heat loss.
3 · Skin and muscle compete for blood
Working muscle needs massive blood flow for oxygen and skin needs massive flow for cooling, and with finite cardiac output the heart must beat faster to serve both.That is why heart rate in heat is 10-20 bpm higher at the exact same pace.
4 · Humidity is the killer
Evaporation depends on how much more water vapor the air can hold, so in high humidity sweat forms but cannot evaporate and drips off without cooling.Wet-bulb 30°C is far more dangerous than dry-bulb 30°C. Heat acclimation (10-14 days) helps via earlier, more, and more dilute sweat, but has limits.