Going out for a run in heat and trying to reproduce exactly the paces we reach in cooler conditions usually ends in frustration, premature fatigue, or a session that turns out much tougher than expected. When the thermometer is playing on the opposing team, part of the energy available is spent cooling the body. That’s why reducing the pace doesn’t mean training worse, but adapting the speed to maintain the stimulus we were really seeking.
Why the same pace costs more
While running, the muscles generate a large amount of heat. To limit the rise in internal temperature, the body sends more blood to the skin and increases sweat production. This mechanism facilitates heat loss, but also forces the cardiovascular system to distribute its resources between the working muscles and the surface of the body. The heart must beat faster to sustain an effort that, at moderate temperatures, would require fewer pulses. As training progresses, the so-called cardiovascular drift can appear: even if we maintain the same speed, the heart rate continues to rise progressively. Humidity adds a significant difficulty. Sweat cools the body when it evaporates, not simply when it appears on the skin. In a very humid environment, that evaporation is less effective. Therefore, 27 degrees with high humidity can be more limiting than a higher temperature in a dry environment, especially if there is no wind and the route is exposed to the sun. A broad analysis from the International Journal of Environmental Research and Public Health on endurance tests confirmed that temperature, humidity, wind and solar radiation jointly influence performance, although the response changes according to distance and the runner’s level. The runners who are fastest are also affected, but they tend to spend less time exposed to adverse conditions.
¿Cuántos segundos por kilómetro debes perder?
No existe una equivalencia universal entre temperatura y ritmo. Dos corredores con la misma marca pueden responder de manera muy diferente debido a su aclimatación, tasa de sudoración, hidratación, composición corporal, edad o tolerancia individual al calor. Aun así, para un rodaje continuo puede utilizarse esta orientación inicial:
| Approximate Conditions | Suggested Adjustment |
| Less than 20 °C and moderate humidity | Normal pace or minimal adjustment |
| Between 20 and 24 °C | Between 0 and 10 seconds per kilometer |
| Between 25 and 29 °C | Between 10 and 25 seconds per kilometer |
| 30 °C or more | Between 20 and 45 seconds per kilometer, or even more |
| Heat accompanied by high humidity and direct sun | Use the upper end of the range |
These figures are only a starting point. Studies conducted with runners show that performance loss can be roughly between 0.1% and 0.6% for each degree the temperature moves away from the most favorable conditions, though the effect heavily depends on the athlete and the context. A runner accustomed to running at 5:00 min/km might need to shift to around 5:15 or 5:25 min/km on a hot, humid morning to preserve a similar physiological load. In especially adverse conditions, even that reduction may fall short.
Effort matters more than the clock
The most useful way to adapt training is to keep the objective and accept that the pace shown on the clock will be different. If the planned session is an easy run, it should still allow you to breathe with control, hold a conversation, and finish with the feeling that you could have continued. On a perceived effort scale from 1 to 10, an easy run should stay roughly between 3 and 4. If maintaining the usual pace requires reaching a 6, we are no longer performing the intended workout, even if the watch shows the right speed. Heart rate can also serve as a reference, but it should be interpreted with some flexibility. In the first minutes it may stay within the expected zone and rise later due to heat accumulation and fluid loss. When the pulse increases while the pace remains or even slows, sticking to the initial speed usually yields more fatigue than benefit. For easy runs, it is wise to reduce the pace from the start. Waiting until we hit the limit forces a larger correction and increases the cost of recovery.
Cómo adaptar las series y los entrenamientos intensos
In quality sessions, the adjustment depends on the objective. If we aim to work at a specific physiological intensity, we can extend recoveries, slightly reduce the number of repetitions, or guide by effort rather than chasing rigid splits. It is also useful to perform the intervals earlier, choose a shaded route, or split a long continuous block into shorter portions. A 25-minute session at a controlled pace can, for example, become three blocks of eight minutes with short recoveries. The stimulus will remain valuable, but the body temperature will have more opportunities to stabilize. Experts from the British Journal of Sports Medicine consider acclimatization the most effective strategy to reduce performance deterioration. Typically several days of progressive exposure are needed to increase plasma volume, start sweating earlier, and improve thermal tolerance. Nevertheless, being acclimated does not make the runner immune to heat.
No intentes compensarlo corriendo más
Si el calor obliga a completar cinco kilómetros en más tiempo, no es necesario añadir distancia para alcanzar la duración o la carga que aparecía en el plan original. En estas condiciones, los minutos de esfuerzo suelen representar mejor la carga que los kilómetros. Un rodaje fácil de 50 minutos continúa siendo un entrenamiento de 50 minutos, aunque recorremos menos distancia. Forzar el kilometraje habitual puede prolongar excesivamente la exposición térmica y dificultar la recuperación posterior. La hidratación ayuda, pero tampoco elimina el impacto del calor. Conviene comenzar la sesión bien hidratado, disponer de líquido cuando la duración y las condiciones lo aconsejen y reponer después lo perdido. Beber más de lo necesario no permite mantener de forma segura el ritmo de un día fresco.
Cuándo debes detenerte
Dolor de cabeza intenso, mareo, náuseas, desorientación, escalofríos en pleno calor, debilidad repentina, torpeza o incapacidad para mantener el equilibrio no son señales normales de un entrenamiento exigente. Ante estos síntomas hay que interrumpir la carrera, buscar un lugar fresco y comenzar a refrigerar el cuerpo. La confusión o la alteración del estado mental requieren atención médica urgente. Correr más despacio cuando hace calor no supone renunciar al entrenamiento. Supone proteger su propósito. El ritmo es solamente el resultado visible; la carga fisiológica es lo que determina cuánto estamos trabajando. En agosto, escuchar al cuerpo puede ser bastante más preciso que obedecer ciegamente al GPS.
