During sport activities core body temperature can rise to extreme levels which impacts performance. It is not uncommon to observe individuals exceeding 40°C (104°F) during high intensity training and competition though these values naturally vary from person to person.
Your Core Body Temperature rises due to physical exertion. Power generation during cycling, running or swimming can be measured in watts. Heat can also be measured in watts since it is the byproduct of power generation and if your body's thermoregulation mechanisms cannot get rid of the excess heat, it can build up to raise your Core Temperature.
To regulate an increasing body temperature, your body sweats and the evaporation process cools the skin through a process called convective heat transfer or convection. Convection is also the reason for feeling much colder when it is windy and why water of 25°C feels much colder than an air temperature of 25°C.
Another thermoregulatory process of the human body is conductive heat transfer or conduction, where heat radiates through solid materials. This principle is also the reason why touching metal surfaces is perceived to have a lower temperature than wood or plastic, even though they have the exact same surface temperature. Similarly, your body diverts blood flow to the skin, which is cooled down by the convective effects mentioned before and uses conduction to release heat and thus cool the blood down. Then the blood is circulated back into the body to cool down your Core Temperature.
So, when your body is actively cooling – it diverts blood flow to the skin so that heat can be released into the surrounding air or water and thus the blood flow is diverted away from the muscles generating the power (heat source).
A useful analogy for this principle is exercising at higher altitudes – at higher altitudes, your FTP (Functional Threshold Power) decreases due to the lack of oxygen. Similarly, when you are overheating, the available oxygen in the muscles is decreased and thus the physical effort is compensated or lowered consequently.
So theoretically, when you overheat, you should actively cool and regulate/lower your output to compensate the blood/oxygen diversion for cooling of your body – but as an Olympic sport researcher told us “yes that is true but that is not how you win gold medals”.
Passive heat stress results in a decrease in peripheral resistance, an increase in cardiac output, and a selective elevation and redistribution of blood flow from the core to the extremities, head, and superficial tissues of the torso. Despite modest increases in aerobic metabolism, this response is predominantly driven by thermosensitive mechanisms, resulting in significant cardiovascular strain even at rest, alongside tissue blood flows *
* J. D. Périard, S. Racinais (eds.), Heat Stress in Sport and Exercise, https://doi.org/10.1007/978-3-319-93515-7_3
Changes in cutaneous circulation can significantly augment heat loss—or minimize dry heat gain—during sports and exercise in hot environments. While resting in a thermoneutral environment, about 0.5 L min-1 of blood (5–10% of cardiac output) is supplying the cutaneous circulation, but during heat stress the cutaneous circulation receives up to 8 L min-1(50–70% of cardiac output) **
** Lossius K, Eriksen M, Walløe L. Fluctuations in blood flow to acral skin in humans: connection with heart rate and blood pressure variability. J Physiol. 1993;460:641–55.