Athletes sent to compete in hot climates arrive days or weeks early, and the reason is physiological rather than logistical. The body makes specific measurable adaptations to repeated heat exposure, and they take time.
Heat limits performance through the circulation
Working muscle needs blood, and so does the skin when the body is trying to shed heat. Both demands draw on the same circulating volume at the same time.
When skin blood flow rises to cool the body, less blood returns to the heart, so each beat pumps less and heart rate climbs to compensate for the shortfall.
That rising heart rate at a given workload is the first sign an athlete notices, and it is why familiar training paces feel far harder in the heat than they do in cool conditions.
Plasma volume expands first
Within the first several days of repeated exposure, the body increases the fluid portion of the blood, which enlarges the total volume in circulation.
A larger volume lets the body supply skin and muscle simultaneously without the same competition, so heart rate at a given pace falls back towards normal.
This adaptation appears early and is also lost quickly, which is one reason acclimatisation has to be maintained rather than achieved once and forgotten.
Sweating changes in timing and composition
An acclimatised athlete begins sweating at a lower core temperature, so cooling starts before heat has accumulated rather than in response to it.
Sweat is also produced in greater quantity and spread more evenly across the body, which increases how much heat can be removed through evaporation.
At the same time the sweat becomes more dilute, because the glands reabsorb more salt before it reaches the skin, which reduces the electrolyte cost of a long session.
The stimulus has to be genuine heat strain
Adaptation responds to raised core temperature, not to time spent outdoors, so easy sessions in warm weather do very little.
Protocols therefore involve sustained work in hot conditions for a period each day, repeated over one to two weeks, with the intensity managed so the athlete can complete the block.
Humidity matters as much as temperature, because evaporation is what removes heat, and in humid air sweat runs off the skin without cooling it.
Why it carries real risk
Deliberately raising core temperature repeatedly is a demanding stressor, and heat illness develops faster than most people expect once thermoregulation is overwhelmed.
Individual tolerance varies widely with body size, fitness, medication and prior illness, which is why supervised protocols monitor core temperature rather than relying on how an athlete feels.
Anyone planning heat training around an event should do it under qualified supervision, since the margin between an effective session and a dangerous one is narrower than in ordinary training.