Training in Extreme Heat — Your Body’s Emergency Protocol

When you work out in heat, your body’s main mission isn’t “go faster” — it’s don’t overheat.

To dump excess heat, blood is sent from your muscles to your skin so you can sweat and radiate heat away. That’s great for cooling, but it means less oxygen and fuel reach your muscles. Your heart tries to fix this by beating faster — that’s why your heart rate is higher even at your normal pace.

Sweat isn’t just water. It’s plasma from your blood, carrying sodium, potassium, and magnesium. The more you sweat, the less plasma you have left, so blood volume drops and muscles get less oxygen. Lose enough sodium and your muscles and nerves start misfiring — cramps, fatigue, clumsiness.

Inside your muscles, the heat makes you burn through glycogen faster and produce more lactate, so you hit that heavy-leg feeling sooner. Meanwhile, your brain starts to slow you down to protect you — reaction time drops, coordination fades, decision-making gets fuzzy.

If your core temp pushes past ~40°C, your brain’s thermostat (the hypothalamus) can’t keep up anymore. That’s when things shift from “hard workout” to “medical emergency.”

In short: training in heat is your body juggling cooling, fueling, and survival — all at once