Running during the hottest part of the day isn’t simply doing the same workout with a more uncomfortable heat sensation. When the environmental temperature is high, especially if there is a lot of humidity and hardly any air flow, the body faces a double demand: producing the energy needed to run and, at the same time, dissipating the heat generated by the muscles. During the run, a large portion of the energy the body produces is not transformed into movement but into heat. To prevent the internal temperature from rising excessively, the body activates two main mechanisms: increasing blood flow to the skin and producing sweat. The problem is that both responses compete directly with the needs of the exercise.
The heart works harder, sweating increases, fluid loss accelerates, and the brain progressively reduces the intensity you can sustain.
The heart has to work harder
When you run, the muscles need to receive blood rich in oxygen. With elevated temperatures, the skin also demands a greater amount of blood to release heat to the environment. The cardiovascular system must attend simultaneously to these two demands, so the heart’s workload increases even if you maintain exactly the same pace. As training progresses, the so-called cardiovascular drift may appear. The amount of blood ejected by the heart with each beat gradually decreases and the heart rate rises to compensate. That’s why a pace you normally sustain at 145 bpm can push you to 155 or 160 on a very hot day without you having accelerated. This drift begins to appear during prolonged exercise and is amplified when heat, dehydration and rising body temperature combine. In addition, it can reduce the available maximum oxygen uptake and turn a run that seems comfortable into a moderate-to-high intensity effort.
Sweating does not necessarily mean you’re cooling down properly
Sweating is the main cooling mechanism during the run, but the body does not cool by producing sweat; it cools when that sweat evaporates from the skin. This difference is especially important when ambient humidity is high. In a dry environment, evaporation tends to be more effective, although fluid losses can be very large without the runner fully realizing it. In humid conditions, the sweat remains on the skin or drips to the ground without adequately fulfilling its cooling function. You can be completely soaked and still continue to accumulate heat. The lack of wind, direct solar radiation, heated asphalt and non-breathable clothing further hinder heat elimination. For this reason, the temperature shown in a weather app does not by itself explain the true thermal load borne by the runner.
Dehydration reduces blood volume
Each liter of sweat lost corresponds to roughly one kilogram of body weight. If the liquid is not replenished, plasma volume decreases, that is, the liquid portion of the blood. The heart then has less blood to send to the muscles and to the skin with each contraction. The consequence is a new rise in heart rate, less effective cooling, and a greater perception of effort. Dehydration can also affect muscle, brain, and respiratory function, and its effects become more pronounced the higher the ambient temperature. A loss close to or above 2% of body weight can compromise endurance performance, although the specific response varies among runners and situations. This does not mean you should drink compulsively. Needs depend on duration, intensity, heat and your individual sweating rate. Drinking far beyond losses can also be dangerous. In short sessions, the runner should start properly hydrated and use thirst as a reasonable reference; in long or especially demanding workouts it is wise to plan fluid replacement on an individual basis.
The muscles burn through their stores first
Heat also alters metabolism. To maintain a given speed, the body may rely more on carbohydrates and accelerate the use of muscle glycogen. At the same time, the elevated muscle temperature alters fiber function and can make it harder to maintain strength and running economy. The result is typically a progressive loss of efficiency. The runner needs to expend more physiological resources to move at the same pace, and the feeling of heaviness tends to appear earlier. Ventilation increases, breathing becomes more demanding and the perceived effort grows even though the clock indicates a seemingly controlled pace.
The brain also compels you to slow down
Heat-related fatigue isn’t determined solely by the heart or the muscles. The central nervous system actively participates in regulating effort. When internal temperature rises and heat buildup threatens the body’s stability, the brain reduces motor drive and increases the sense of difficulty. This mechanism acts as a protection. Before the body reaches a critical situation, there arises an almost irresistible urge to lower the pace. Motivation, concentration and tolerance to effort decline. At high intensities, coordination, decision-making and the ability to respond to pace changes can also deteriorate. That’s why it’s not realistic to expect the same marks on a cool day as on a day of extreme heat. Trying to hold habitual paces at all costs can turn a productive workout into an excessively demanding session.
Performance decreases even if you’re well trained
The magnitude of performance loss depends on temperature, humidity, solar radiation, wind, race duration and the level of acclimatization. Trained runners are not immune either. They tend to sweat earlier and cool better, but they can also generate a lot of heat when running at higher speeds. Acclimation, achieved through repeated progressive exposure to heat for about one or two weeks, improves the physiological response. The body begins to sweat earlier, increases plasma volume, reduces the concentration of salts in sweat and better controls temperature and heart rate. Still, acclimating does not eliminate the risk nor permit ignoring extreme conditions.
When you should stop
Severe headache, dizziness, nausea, chills despite the heat, disorientation, loss of coordination, abnormal weakness or confusion are signals that should not be interpreted as a simple lack of form. In the face of these symptoms you must immediately stop exercising, seek shade, start cooling the body and seek help when the disturbance is significant. In the hottest hours, reducing pace does not mean training poorly. It means understanding that the body is allocating a substantial portion of its capacity to maintaining internal temperature. Whenever possible, moving the session to the early morning or the end of the day allows you to complete the workout with less cardiovascular stress, less dehydration and a much higher quality.