In an air temperature of 10°C/50°F, our runner can finish a marathon in 3:30. For each 5°C/9°F increase in air temperature, that runner needs an extra 5 minutes to finish.

Marathon performances slow as environmental temperatures increase. An air temperatures of 10°C/50°F is ideal for runners finishing in more than 3 hours.

Research shows that for a runner who would normally finish in 3:30 at 10°C/50°F, each 5°C/9°F rise in environmental temperature (up to 25°C/77°F) results in an additional 5 minutes needed to finish.

In other words, at 25°C/77°F, that 3:30 finish turns into a 3:45 – a 7% slowdown. Faster runners are affected less by rising temperatures, and slower runners are affected even more. 

Rising core body temperature

Why such drastic slowing in temperatures that most people consider quite mild? Even in these relatively cool environmental temperatures, core body temperature continually increases because of the high aerobic demands of the marathon. Over 80% of the energy exerted in marathoning is transferred as heat to the body core. This means that the body must dissipate an enormous amount of heat to keep functioning. In warmer air temperatures, it is more difficult for the body to cool itself. Thus, core body temperature rises and performance drops as even more blood is diverted from power-producing muscles to be cooled by the skin.

Mitigating the effects of heat

There are two primary ways to mitigate the effects of heat on your marathon time: heat training before the race and preemptive cooling during the race. The CORE sensor plays an important role in both strategies.

Heat training involves safely elevating the core body temperature during workouts. This helps build and maintain blood plasma levels which, in turn, increases haemoglobin and total blood volume. This means that as blood is diverted to the skin for cooling, ample blood remains to provide oxygen to the muscles. The CORE sensor is an ideal tool for heat training.

During races, the CORE sensor delivers real-time core body temperature to athletes via their smart watch. This allows runners to monitor their core temperature and take cooling strategies before their performance suffers. For example, in events starting in very cool morning air, many runners begin races wearing an additional layer of clothes, and then shed that top layer as they warm. A runner wearing a CORE sensor may notice that their temperature is rising before they're consciously aware that their extra clothing is not needed. This reminds them to shed the extra layer early and to slow the inevitable rise of their core temperature.

Another typical cooling strategy in marathons is pouring water over the head. And long distance triathlons and trail races often offer ice cubes to be placed under the hat or in clothing. Monitoring core temperature alerts racers when they should take such measures, possibly even before the athlete consciously feels overly hot.

In short, the CORE sensor can help athletes prevent some of the slowing that comes in marathons run in temperatures warmer than ideal.

Vihma, Timo. (2009). Effects of weather on the performance of marathon runners. International journal of biometeorology. 54. 297-306. 10.1007/s00484-009-0280-x.