Let's try to understand what happens to the heat released by the body during the physical activity. The reason why we should do this is because there are a lot of sportsmen out there who surprised what happens to their body while they exercise, so here it is.
Almost all the energy released by the metabolism of the body and which is taken from the nutritive substances we eat are finally converted into heat. This is also valid for the energy which produces the contract of the muscles and here is why. First of all, the maximum efficiency when it comes to transforming the energy you get from food into muscle work is approximately twenty – twenty-five per cent, the rest of the energy being converted into heat during the intracellular chemical reactions. Secondly, almost all the energy which is supposedly to produce this muscle work is also transformed into heat because it is almost entirely used for fighting against the resistance which appears when the muscles and the articulations move, for fighting against the friction forces of the blood running through the veins and for other similar effects. All these convert the energy of the muscles into heat.
If we take into consideration the potential of the consumption of oxygen increasing more than twenty times with a trained sportsman and the proportional increase of the consumption of oxygen and the heat released into the body, we can easily understand that there are great quantities of heat which are released into the internal tissues of the body during the physical activities. Moreover, if this large quantity of heat is produced during a hot and dry day, the sportsman not being able to remove the heat by sweating, the sportsman can suffer from a thermal shock.