The influence of heat on your running performance

Reposted: May 2023

It is warm. So any self-respecting running magazine warns you to be mindful of the heat. ProRun does that too. That’s right. If you start too fast, you can be overcome by the heat and suffer health problems. A summer temperature affects your running performance anyway. The most important thing is that you know what you can and can’t do in hot weather, adjust to it and stay healthy.

Heat is so important in sports that the Dutch government made a large amount available for research into the influence of heat on athletes in view of the postponed Olympic and Paralympic Games in Tokyo. The research project is very appropriately called ‘Thermo Tokyo: Beat the Heat’.

The temperature in Tokyo in August is (well) above 30°C and the humidity is high. These conditions lead to reduced sports performance and potential health problems. The goal of the research project is to minimize loss of performance and to ensure that top athletes can safely perform to the maximum in a hot and humid Tokyo. To achieve this, a personal heat profile of athletes is drawn up with a personal plan of action. The insights gained for athletes can also be used for working conditions in numerous professions and thus serve a social interest.

This is not for ordinary runners, but in this article we provide a number of things worth knowing that will probably be of use to you. For those who want to know more, we refer to our book Run with Power!.

Your body and the circumstances

It’s good to realize that your human engine only uses 25% of its energy to provide running power. That means that 75% of the carbohydrates and fats you burn while walking are converted into heat that your body has to get rid of.

That 25% applies to top athletes who have taught their bodies to use energy efficiently through training. Ordinary runners run less efficiently, the percentage is lower than 25%, and therefore have to dissipate more energy as heat from their body.

When you run faster, you use more energy and produce more heat. That’s why you often see competitive athletes still running in singlets and shorts, while recreational users think it’s chilly and wear a sweater and long running tights.

Wind provides cooling. That works well in the summer, but we notice it best in the winter by the wind chill. Strong wind makes it quite cold. This is because the wind blows away the insulating layer of air around the skin.

In the summer it feels warmer when the humidity is high. Your body loses heat through perspiration. The sweat on your skin evaporates, cooling your body. At high humidity, it is more difficult for sweat to evaporate, you cool down less, your body temperature and your heart rate rise, and you have a hard time. Unless you adapt to the circumstances.

It is therefore about the temperature and the humidity, which can be summarized in the so-called wet bulb temperature. There is a significant relationship between wet bulb temperature and realized running times. The effect of the temperature on your time depends on the distance. The influence of temperature on your marathon time is greater than on your 5 km time.

In addition to the heat balance and sweat loss, another factor is of great importance. At tropical temperatures and high relative humidity, you no longer lose heat through sweating. The so-called heat stress index is a measure of this. If it approaches 1, you can just lose the heat you produce to the environment. Above 1 you will be in trouble. Under such conditions, many runners would succumb and competitions should be banned.

You can also get into trouble below the heat stress index 1. The exemplary figure Marathon Man in our books weighs 70 kg and takes 3:30 to complete the marathon. His net sweat loss at a wet bulb temperature of 30°C is then 1 liter per hour, so net 3.5 liters over the entire marathon. Because sweat also drips from the body and absorbs into clothing, the gross sweat loss is two to three times more, 7-10 liters. To stay out of trouble, someone’s fluid loss should not exceed the limit value of 5% of body weight. The rise in body temperature must also remain below 1.5°C. In order not to get into trouble, Marathon Man has to run the marathon a lot more slowly under these circumstances.

Better runners produce more heat. There is a relationship with the anaerobic threshold power ADV, the power that you can just sustain for an hour. But there is also a relationship with weight. Small trained runners are therefore less affected by the heat than large men who should actually lose a few kilos.

Women are usually smaller and lighter than men, but are generally more affected by the heat. This is because women naturally have a higher percentage of fat (or maybe you prefer to read that women have relatively less muscle mass?).

It also plays a role whether you are walking on black asphalt in full sun or in the shade of the forest. Direct rays of the sun and reflection warm up your skin more.

It also matters whether you are acclimatized to the heat. With the Rotterdam marathon, for example, it can suddenly feel warm in April at a temperature that we do not consider particularly high in the middle of summer. The top model watches from Garmin keep for the calculation of the VO₂ max take the temperature (and the altitude) into account. If it is above 22°C for several days, the calculated VO decreases₂ max and the times attainable according to the watch at distances change accordingly. We do note that Garmin is too optimistic with these times and you better use the tables from the book for this Run with Power! can use.

Temperature and Marathon Man

The foregoing makes it clear that quite a few aspects require attention if you want to run fast paces in warm weather. Each distance also has its own temperature at which you perform optimally. Roughly speaking, the temperature is lower as the distance is longer. And the lower your performance index, the higher the optimal temperature or you should dress warmer.

Complicated. Nevertheless, we can provide you with information. We do that with Marathon Man as an example.

The table below shows the achievable times for Marathon Man, depending on the temperature and distance. In the graph you can see that the times also get worse when it is colder than optimal. T_note stands for wet bulb temperature.

With the calculator at www.HetGeheimvanHardlopen.nl or at ProRun you get a good indication of the influence of the temperature on your performance. You could take that as a target time for a race on a hot day, not forgetting the other aspects mentioned in this article.

Handy running app

The app is available for iOS and Android Good To Run a handy tool to determine what you do best in the current weather conditions. The free version looks 24 hours ahead. The paid one does a whole week. The app shows all kinds of weather data for the position where you are, including the wet bulb temperature (WBGT Wet-Bulb Globe Temperature).

The flag colors you see are according to the definition of the United States Army (USA). Yellow stands for “Moderate Risk of Hyperthermia. Heat-sensitive runners should slow down the pace.”
As you can see, the heat risk can vary from hour to hour. If you know what you can do, it is possible to personalize the criteria for the flag color (and therefore the risk/advice).