Exercising in the heat

The body has very good mechanisms for controlling its core temperature which needs to be in a very narrow range (36.5-37.5 degrees).


During exercise, body heat is produced by both internal sources – through muscle activity and metabolism – and by external sources – the environment. It is critically important that we are able to release heat in order to avoid tissue damage within the body and to be able to continue exercising.


At rest, in a cooler environment, we mainly lose heat through convection (the transfer of heat to moving air or water) and radiation (transfer of heat by rays without any contact), but during exercise we largely rely on the evaporation of sweat to control our temperature. Sweat evaporation is affected by humidity and so in hot and humid conditions the ability to lose heat is reduced.




The amount of heat generated during exercise is dependent on a number of factors – the length and intensity of the exercise, as well as body mass (the more mass, the more heat generated). There also appears to be innate or genetic factors that make some people more resilient or more susceptible to heat related illnesses during exercise. If environmental factors are hot and humid and exercise is prolonged, then there is a risk of a significant rise in core body temperature. It is not unusual for core temperatures to increase up to about 40 degrees in endurance athletes. Core temperatures over 41 degrees can only be tolerated for short periods and core body temperatures above that lead to tissue damage, heat stroke and organ failure if left untreated.


You can lose up to 2 litres of sweat an hour when exercising!


While performing heavy exercise in hot conditions an average-sized person can lose up to 2L per hour in sweat alone. If fluid is not replaced then dehydration can ensue. In a situation where the cardiovascular system is already under huge demand then dehydration can have a significant impact. It reduces exercise performance, and reduces the ability of the body to cool itself, thus increasing the risk of further heat stress in a vicious cycle.


Tips for endurance exercise in the heat:

  1. Acclimatise yourself to exercise in the heat as much as possible before an event or race in hot conditions. This usually takes 2 weeks of being exposed to living and training in hotter conditions.
  2. Reduce your pace – this reduces heat production, and means it is more likely you’ll maintain an appropriate core temperature.
  3. Maintain hydration – the current advice is to drink to thirst. Forcing down too much fluid can result in other problems such as low blood sodium (hyponatraemia). Practice your hydration techniques in training, measure your pre- and post-exercise weights to understand how much fluid you tend to lose in different conditions.


Written by Dr James Lawrence

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