How heat affects running pace

If you’ve ever gone out for an easy run on a hot day and wondered why your pace suddenly feels impossible, you’re not imagining it. Heat has a measurable, predictable effect on running performance, and even well-trained runners slow down significantly as temperatures rise.

Understanding why this happens - and how to adjust - can help you train more effectively and avoid overreaching in hot conditions.

Why heat slows you down

The main challenge of running in the heat isn’t your muscles—it’s thermoregulation.

When you run, only about 20–25% of the energy you produce turns into forward movement. The rest becomes heat. In cool conditions, your body can dissipate that heat efficiently. In hot or humid conditions, this process becomes far less effective.

To prevent core temperature from rising too high, your body:

Increases skin blood flow for cooling
Increases sweating
Diverts blood away from working muscles

This creates cardiovascular strain, forcing your heart to work harder at the same pace. As a result, pace must drop to keep effort sustainable.

Research shows that elevated core temperature is one of the primary factors limiting endurance performance in the heat (González-Alonso et al., 2008).

The heart rate drift effect

One of the most noticeable signs of heat stress is cardiovascular drift.

At a constant pace in hot conditions:

Heart rate gradually rises
Stroke volume (blood pumped per beat) decreases
Perceived effort increases

This means a pace that feels easy early in the run can become unsustainable later, even without speeding up. Studies show heart rate can rise by 5-15 beats per minute during prolonged exercise in the heat, purely due to thermal strain (Coyle & González-Alonso, 2001).

This is why pacing by effort or heart rate (rather than pace alone) is critical in hot weather.

How much does heat affect running pace?

While individual responses vary, research and field data show consistent trends:

Performance begins to decline above ~10-12°C (50-54°F)
Losses accelerate above ~20°C (68°F)
At 30°C (86°F), runners may slow by 5-10% or more

Marathon studies show finish times worsen progressively as temperature rises, with the fastest runners often experiencing the greatest relative declines due to higher absolute heat production (Ely et al., 2007).

Humidity further compounds the issue by limiting sweat evaporation, making cooling even less effective.

Perceived effort increases faster than pace

A key feature of heat stress is that perceived effort rises disproportionately compared to pace.

You may be running slower, yet feel like you’re working harder. This happens because the brain integrates signals from core temperature, skin temperature, heart rate, and hydration status. As thermal strain increases, the brain subconsciously reduces motor output to protect the body - a concept known as the central governor model (Nybo & Nielsen, 2001).

In practice, this means forcing pace in the heat is rarely productive and often counterproductive.

Heat acclimation: adaptation is real

The good news is that the body adapts to heat exposure.

After 7–14 days of heat acclimation, runners typically experience:

Lower heart rate at a given pace
Earlier onset of sweating
Increased plasma volume
Improved thermal comfort

These adaptations improve heat tolerance but do not eliminate performance losses entirely. Even acclimated athletes still slow down in hot conditions compared to cool ones (Périard et al., 2015).

Practical pacing adjustments for hot runs

To train safely and effectively in the heat:

Expect pace reductions of 3–10% depending on temperature and humidity
Use effort or heart rate instead of pace targets
Start slower than you think you need to
Shorten hard intervals or increase recovery
Hydrate appropriately, but don’t rely on fluids alone to “fix” heat stress

Easy runs should stay easy - even if that means significantly slower pacing.

When heat becomes dangerous

Heat illness risk increases when pace, hydration, and recovery aren’t adjusted. Warning signs include:

Dizziness or confusion
Chills or goosebumps despite heat
Sudden drop in performance
Nausea or headache

If these occur, stop running and cool down immediately.

Key takeaway

Heat slows running pace because it increases cardiovascular and thermal strain—not because fitness suddenly disappears. Slower pace in hot conditions is a normal physiological response, not a weakness.

Respect the conditions, adjust expectations, and remember: fitness gains still happen even when the pace is slower.

References

Coyle, E. F., & González-Alonso, J. (2001). Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews.
Ely, M. R., et al. (2007). Impact of weather on marathon running performance. Medicine & Science in Sports & Exercise.
González-Alonso, J., et al. (2008). Direct evidence for the role of the skeletal muscle pump in human cardiovascular regulation. Journal of Physiology.
Nybo, L., & Nielsen, B. (2001). Perceived exertion is associated with altered brain activity during exercise with progressive hyperthermia. Journal of Applied Physiology.
Périard, J. D., et al. (2015). Heat acclimation and exercise performance. Sports Medicine.