Every summer, the same concern comes up among many runners: “Why am I slower even though I’m fit?”
The explanation is simple: heat places additional strain on the body. As temperatures rise, the body must use more resources to keep its core temperature within the range required for the organs and muscles to function properly.
In other words, you’re not performing worse. Your body is simply working harder to cool itself down. The heart has to pump more blood to the skin to dissipate heat; heart rate increases, perceived exertion becomes greater, and fatigue sets in more quickly. Under these conditions, trying at all costs to maintain the same pace as in spring or autumn is often counterproductive.
The goal, therefore, is not to fight the heat, but to adapt your training so you can continue to improve while staying healthy.
When the outside temperature rises, the body activates various mechanisms to prevent overheating
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increased sweating
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increased blood flow to the skin
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increased heart rate
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increased energy expenditure
Some of the energy that could otherwise be devoted to performance is redirected towards maintaining a stable body temperature. This is why a pace that feels easy at 15°C can become much more demanding at 30°C, even if your fitness level hasn’t changed.
Temperature, however, is not the only factor to consider. Humidity also has a strong influence on the body’s ability to cool itself.
When we sweat, the body releases heat through the evaporation of sweat from the skin’s surface. But when the air is already saturated with moisture, evaporation becomes less effective. Sweat flows more freely, but it does not cool the body as efficiently.
As a result:
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heart rate rises further
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perceived exertion increases
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body temperature rises more quickly
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the risk of overheating and dehydration increases
This is why running at 25°C in high humidity can sometimes feel more difficult than running at 30°C in dry air.
Under these conditions, it may be necessary to:
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slow down more than the temperature alone would suggest
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shorten the workout
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pay closer attention to hydration
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prioritise short, high-quality sessions
More than temperature alone, it is often the combination of temperature and humidity that determines the actual difficulty of a summer workout.
Heat is also often accompanied by poorer air quality. Episodes of ozone pollution are particularly common in summer, especially in urban and suburban areas.
During endurance exercise, breathing rate increases significantly: we breathe faster and more deeply, which increases the amount of pollutants we inhale. For some people, this can lead to irritation of the respiratory tract, reduced performance, increased shortness of breath, coughing, headaches, or difficulty breathing.
When temperatures are high and pollution levels are significant, it may be best to
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exercise early in the morning, when ozone concentrations are generally lower
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choose wooded or natural areas away from traffic
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reduce the intensity of your workout
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postpone the outdoor session or replace it with an indoor activity
As with heat, the goal is not to train at all costs, but to choose the most favourable conditions to protect your health and maintain a sustainable exercise routine.
There is no one-size-fits-all formula. Every runner responds differently depending on their fitness level, acclimatisation to heat, hydration, and age.
However, a few guidelines may be helpful
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20–25°C: slightly adjust your pace if necessary
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25–30°C: reduce your pace by approximately 5–10%
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Over 30°C: reduce intensity, shorten your workout, or run during the coolest hours of the day
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In high humidity: adjust your effort further
In practical terms, a runner who usually runs at an easy endurance pace of 6:00/km might comfortably run between 6:15 and 6:30/km on a very hot day. This adjustment is not a sign of weakness. On the contrary, it is one of the best ways to maintain a consistent training load.
In hot weather, the pace displayed on your watch becomes less relevant. How you feel and your heart rate often become more reliable indicators.
If, at your usual pace:
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your heart rate is noticeably higher
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your breathing becomes more difficult
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an easy workout suddenly feels demanding
it is best to slow down, reduce the intensity, or shorten the run. The goal is to maintain the desired physiological intensity rather than adhere to a specific pace at all costs.
Heat particularly affects long, continuous efforts. However, there is no need to abandon quality sessions.
Certain workouts remain particularly beneficial
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technique work
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running drills
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strides
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short sprints
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short intervals with sufficient recovery
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strength training
These formats help maintain neuromuscular qualities, coordination, and speed while limiting the build-up of heat and fatigue.
The most demanding workouts in hot weather are generally
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long runs
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sustained efforts lasting 20 minutes or more
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high-volume workouts
During heat waves, it is often better to reduce your training volume by 20–30% rather than follow your original plan exactly. It is better to finish a session feeling that you still have some energy in reserve than to push yourself into excessive fatigue that will affect your performance in the days that follow.
Certain habits can help reduce the impact of high temperatures
Whenever possible, opt for:
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early-morning runs
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evening outings
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shaded or wooded areas
The goal is not to drink excessively, but to gradually replace the fluids you lose. Waiting until you feel thirsty can be a sign that you are already behind on hydration. After the hottest or longest workouts, it may also be helpful to replace some of the minerals lost through sweating.
Summer is not always the best time to pursue personal bests.
It can be an ideal time to build:
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endurance
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technique
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strength
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adaptability
Certain symptoms require you to stop exercising immediately
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dizziness
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severe headache
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nausea
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chills
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confusion
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unusual weakness
These symptoms may indicate heat illness. Stop exercising and cool down as quickly as possible.
Summer is not the time to prove your fitness by maintaining the same pace at all costs. Runners who make progress over the long term are often those who know how to adapt their training to the demands of the moment.
Accepting the need to slow down, temporarily reduce your training volume, listen to your body, and prioritise quality over quantity does not mean you are regressing. On the contrary, it is often the best strategy for continuing to improve while protecting your health.
When it comes to training, wisdom sometimes means doing a little less today so you can do more tomorrow.