Running performance, once the effort exceeds a short distance (like a sprint), declines significantly and almost universally as soon as the temperature exceeds a moderate optimal threshold, generally situated around 15-16°C.

noli's read

The consensus among practitioners is that heat negatively impacts aerobic efficiency, even over distances as short as 5 km. A meta-analysis published in Frontiers in Physiology (2021) confirms this trend, noting that performance declines become significant starting at 15°C across a wide range of distances. The underlying physiological mechanism, often cited by the community, is cutaneous vasodilation, which redirects a portion of blood volume from the muscles to the skin to regulate internal temperature, thereby reducing available oxygen supply. While the thermal impact is widely proven for endurance, the idea that a specific "tipping point" exists (around the mile or 5 km) remains an empirical observation that varies according to an individual's training level and heat acclimation. The "beneficial" aspect of heat only holds for pure sprint events, where reduced air density and improved muscle elasticity take precedence over cardiovascular constraints. The community strongly converges on the idea that heat is a limiting factor as soon as the aerobic pathway becomes predominant.

the sources

1. 1Périard JD, Eijsvogels TMH, Daanen HAM 2021
2. 2Tyler CJ, Reeve T, Hodges GJ, Cheung SS 2016
3. 3Racinais S, Alonso JM, Coutts AJ, et al. 2015
4. 4r/AdvancedRunning