Chapter 1: Reasons for cooling during endurance training and race (in both warm ánd cool conditions)
You might consider cooling your body when temperatures are high. But, did you know that the optimal temperature for a marathon runner is 6.2°C (43°F) for men and 6.8°C (44°F) for women? And that performance decreases with every degree the temperature rises? When temperatures are around 26°C (79°F) the performance decreases by 17.7% for men and 12.4% for women. So, cooling your body, also in cool temperatures, makes sense. (*2)
Well trained endurance athletes can attain core temperatures of even above 40 degrees C (104°F). But during exercise this can lead to:
- psychological symptoms like confusion / aggressiveness / irrational behavior / altered consciousness.
- physiological symptoms like muscle cramps / nausea / headaches / weakness / reduced muscle coordination / diarrhea and even full collapse. (*1):
Exhaustion is the limiting factor in endurance sport. It was thought that exhaustion in hot conditions was a result of circulatory failure (a reduction in cardiac output and muscle blood flow). But recent studies have shown that your core temperature is the limiting factor. (*3)
For every degree your body’s internal temperature rises, your heart rate increases by about 10 beats per minute. That natural reaction places added strain and stress on your heart. (*6)
It has been shown that muscle power output, is reduced by elevations in core temperature. (*11)
Around 2 million results from 6 major marathons were compared. It was found that times began to get slower and DNF’s (Did Not Finish) already increased at around 10°C (50°F). At a temperature of 20°C (68°F) average times would decrease by almost 19 seconds per KM (30 seconds per mile) for an athlete running a 3:15 marathon. This means more then 13 minutes over the entire marathon! (*12)
In another study the effects of weather conditions on male performance during the Boston Marathon from 1897 to 2018 were analyzed. A total of 383,982 observations from 244,642 different finishers were analyzed using Generalized Additive Mixed Models. All runners including annual top 100 and top 10 finishers were considered. Temperatures, when increasing by 1°C (1.8°F) was related to worsened performance for all groups; it took them almost 2 minutes more to complete the marathon. (*13)
The science is clear: heat hurts endurance performance, even in cool conditions.
In warm conditions though, hyperthermia is more likely to appear. Hyperthermia is defined as an increase in one’s core temperature above 38 °C during moderate-intensity exercise. There is a growing body of literature showing decreases in performance during submaximal exercise with hyperthermia. Exposure leads to problems such as the stretching of muscle fibres, glycogen depletion, muscular ischaemia, oxygen radical release that may induce oxidative stress, cardiovascular strain, perceptual discomfort and central nervous system dysfunction. As a result, performance decreases when the total amount of cognitive resources are insufficient for both completing a task and combating hyperthermia (core temperature above 38 °C (100.40°F)).
In practice this is what happens: when one’s core temperature increases, first an improvement in cognitive performance appears. But when reaching 37.8°C (100°F) the improvement disappears. Thus, heat strain overloads one’s cognitive capacity when his or her core temperature exceeds this 37.8°C (100°F) threshold, as a result, performance decreases. (*19), (*20)
While the exact reason for reduced performance varies, it has to do with high brain temperatures when core temperature is high. This creates down-regulation of performance, fluid balance offsets, and thermal strain/hyperthermia reducing metabolic efficiency. This all causes peripheral / central fatigue, and stressing the cardiovascular system. (*14)
During exercise in a hot environment, metabolic heat produced by the exercising muscles is transported by the circulating blood to the surface of the body where it is released to the environment, either by radiation and convection or by evaporation of sweat. (*14)
The effect of high ambient temperatures on exercise performance is most evident in prolonged submaximal exercise (more than during short-duration exercise). (*14)
Overall, there is a clear impact of the heat on performance in endurance sports. It leads to an increase in body temperature, which the results tell us leads to a decrease in Vo2Max and an increase in perceived exertion by comparison to cooler conditions (Periard et al., 2021). This results in a decrease in running speed, including in ultra runners (Parise and Hoffman, 2011), likely due to the athlete making the decision to stop (in constant work rate studies) or decrease their power output/speed, as seen in the cycling studies and running research respectively (Periard et al., 2021). (*5)
So; there are very clear reasons why reducing core temperature for endurance athletes during exercise is crucial.