--- title: "Why is running in the heat so hard?" published: 2015-07-20 updated: 2026-08-04 source: "https://dev-abp.massivoai.com/en/articles/por-que-correr-en-calor-cuesta-tanto" --- # Why is running in the heat so hard? ***Anyone who has tried to run in hot weather knows that high temperatures make everything more difficult. But why is it so hard for us to run and perform well in hot and humid climates? What happens in our body? Is it dangerous?*** Our body is a machine, and like any machine it produces heat. The problem is that up to 80% of the energy we have is lost as heat, and only 20% is converted into movement. The more intense the physical activity, the more heat is generated, and that is a problem for the human body machine, which is designed to function within a very limited temperature range: 35-42 degrees Celsius. When we run a race we do it at high intensity because we want to reach the finish line as quickly as possible. This causes metabolism to increase (and therefore heat production) up to 25 times above resting levels. At this rate the body would heat up 1° every 5-7 minutes, and within about half an hour we would inevitably collapse. Fortunately the body has 4 different ways to lose that heat it generates and thus stay within the allowed limits. Those ways are: - Radiation: Heat is transferred through electromagnetic waves. If the environment around the body is cooler, heat is lost; if it is hotter, heat is gained. - Conduction: Direct transfer of heat from one body to another through molecular contact. For example, if the asphalt is hotter than the body, stepping on it will transfer that heat to us. - Convection: Heat transfer by the movement of a gas or liquid. For example, wind sweeps away the hot air molecules surrounding the skin and produces a cooling effect. - Evaporation: The water that evaporates from the skin and in the respiratory passages is the most efficient way to lose heat during exercise. These 4 ways work wonderfully when the weather is cool, they prevent us from overheating and help us maintain good performance. The problem comes when we run in hot climates, like those in Limón and Puntarenas. As temperature increases, the ability to lose heat through the first three mechanisms decreases. In fact, if the temperature exceeds 33°, not only do we not lose heat, but we gain heat through radiation, convection, and conduction. Under those conditions the only method we have left to keep body temperature within the acceptable range is evaporation. So we start to sweat more, to evaporate more and cool down. But if we add high humidity in the environment to the high temperature, we lose the fourth and last way we had to lose heat. The sweat does not evaporate but drips from our skin and falls to the ground. Sweat that falls to the ground is a waste, because it is not the sweat itself that causes cooling, but its evaporation. If it drips without evaporating, we are dehydrating and we do not lose heat. High temperature and humidity is what we commonly have to face in races like Sol y Arena and Reto Jacó Extremo, and that is why they are so difficult. Performance will never be as good in a hot and humid climate as it would be in a cool one. In hot and humid climates: - Less blood reaches the muscles. Heart rate increases but does not compensate for the reduction in blood volume due to sweating. Less blood in the muscles means less oxygen and less work capacity = performance drops. - The lactate threshold is lower. That is, running at the same submaximal speed we accumulate more lactate and reach more quickly the point where it spikes. Lactate is not exactly an enemy as has always been believed, but factors associated with lactate accumulation do reduce performance. - If the temperature reaches 39-40°, it begins to interfere with muscle activation from the brain, reducing the number of muscle fibers recruited. Fewer muscle fibers recruited = performance drops. - More sweat equals dehydration if the lost fluid is not replaced. Dehydration alters several physiological processes that result in lower performance. So don't feel bad if you ran 5 minutes slower than normal in Sol y Arena, there are reasons that justify it. But apart from performance, running in hot and humid climates can be very dangerous, to the point that a person can lose consciousness and even die if they don't take certain precautions. We recommend you read our Tips for running in hot climates and surviving the attempt. Sources: McArdle, Katch and Katch, *Exercise Physiology* (2007); Noakes, *Lore of Running* (2001); Daniels, *Daniels Running Formula*(2005) A Buen Paso uses [Massivo](https://massivoai.com), the event-registration platform that powers this article.