The effect of heat on exercise

Heat: Everyone knows that when there’s too much of it, things can get very uncomfortable, especially if there’s an accompanying high humidity. However, a high temperature is also something that needs to be considered if you are training under these conditions. Besides the general notions to drink plenty of fluids and stay hydrated, excessive temperatures can also adversely affect your performance, so any benchmarks or goals you set for yourself need to be adjusted depending on the conditions.

Take, for example, the route that I have been running for the past several weeks. During that time, I’ve kept records of my run times along with the conditions that day. (Specifically, the temperature, dew point and relative humidity)

On days where the temperature was above 25C my run times were averaging between 45-46 minutes. On days where the temperature was 20C or lower, average times decreased to around 43 minutes. That’s roughly a 4-6% decrease in time when going from 25C to 20C, or conversely, a 4-7% increase in times when going from 20C or below to 25C or higher.

The most recent example were two runs I did this week. On Tuesday, the conditions were 26C and 51% RH, and my completion time was 45:35. Today, the temperature was only 18C and RH was 83% and my run took only 42:53. Despite being faster, today’s run also felt somewhat easier (perhaps somewhat of a placebo effect) as I was able “push” myself harder without feeling fatigued. This was despite the fact that the overwhelming majority of my runs during the past 8 weeks have taken place at temperatures 25C or higher.

My experiences tend to correlate with the studies done. Though I can’t remember the source, I remember reading that 15C was the “ideal running temperature”, with run times expected to increase the further you moved away from this point. Obviously, some of this can be mitigated with physical/mental training and depends on the individual, but these provide some good guidelines.

However, there’s a definite physical aspect to it. Just like the CPU in your computer can run at a faster speed when it’s cooler, so can your body. At higher temperatures, your cardiovascular system must work harder to rid the body of excess heat to maintain a proper core temperature. This means that less energy is “available” for movement and is the reason why you tend to fatigue faster in the heat. (At extremely high body temperatures, the chemical reactions and enzymes necessary for bodily functions are disrupted, possibly leading to death)

Conversely, at extremely low/cold temperatures your body must work harder to generate heat to maintain a proper temperature, with the same effect: less energy is available for movement. 15C is perhaps the “ideal” point where the body has the least amount of work to do to maintain proper core temperatures during running, though that value almost certainly differs amongst individuals.

Thus, during training, if you keep a record of your running times, you should also keep a record of the weather conditions so that times can be taken into context.

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