Physicality of cooling

As human beings, we dissipate a huge amount of energy through our cooling mechanism to keep our core body temperature within a constant range, a body mechanism called homeostasis. Internal metabolic heat production must be balanced by an equal rate of net heat dissipation to ensure a rate of body storage, or heat balance, of zero.

As part of this mechanism, when our body is overheated, our blood flow will be diverted from the muscle to the skin for the cooling down mechanism. For athletes, that means that our body prioritizes constant body temperature to muscle performance. Therefore, when the former dimension is not properly managed it directly affects the performance of the athlete.

Cooling down mechanisms 

There is a variety of cooling down mechanisms both autonomic and behavioral which are activated to tackle the production of heat when an effort occurs. A great part of it goes through the skin heat transfer in various forms; conduction, convection, and radiation. 

  • Conduction relates to the transfer of heat through direct contact between the skin and a solid object, i.e. happens when a colder/hotter material enters in contact with the skin. Its effects are minimal in comparison to the two other ones.  
  • Convection is defined as the transfer of heat to air or water. This convection depends on the airflow speed and therefore is much higher on a bike than during a walk under the same conditions. In the water, induced convective heat loss is mostly dependent on the water’s temperature. Additionally, the effects of convective heat loss are higher in the water than in the air. 
  • Radiation is defined as the electromagnetic energy transfer between a relatively cool and warm body. The most intuitive example is the feeling of heat from the sun rays, quite strong when walking out of the shadows in a cold and sunny winter day. The color of the clothing also influences the radiation.

Additionally, there are other heat exchange mechanisms that could happen either simultaneously or consequentially to skin heat transfer.

  • The respiratory heat exchange occurs when breathing. It results from two combined actions: the hear transfer from the air to the lungs, and the evaporation from the respiratoy track. 
  • The evaporation from the skin surface, due to sweating, is considered the most powerful autonomic response to heat. Unlike the other mechanisms, it is the only heat dissipation mechanism available when air temperature is equal or higher than skin temperature. 

Evaporative heat loss depends on the surface of sweating skin (that’s why clothes reduce body cooling), airspeed, and sweat saturation levels, also known as the skin’s wittedness: the fraction of the skin surface that is covered in sweat. The greater level of skin wettedness, the greater rates of evaporation. Trained and acclimated athletes show higher rates of maximum skin wettedness, so they have a larger capacity to regulate their body through evaporative heat loss. 

When an individual does not manage to keep its heat balance and has a heat surplus, then the body complements the effects of the previously explained mechanisms by diluting the blood vessels at the surface of the skin, in a process also called vasodilation. The higher amount of blood at the surface of the skin allows a better and quicker propagation of those cooling mechanisms. 

The better the physiological ability to make use of those cooling mechanisms, the better the heat balance. Of course, these body's autonomic responses to heat situations could be improved and trained. However, they can also be complemented by more behavioral actions. Check out our article on the cooling strategies on our blog to find out more!  

References

  1. Flouris, A. D. (2019). Human Thermoregulation. In J. D. Périard, S. Racinais (Ed.).  Heat Stress in Sport and Exercise (pp. 3-27). Switzerland, Springer Nature Switzerland AG Retrieved from https://doi.org/10.1007/978-3-319-93515-7_1 
  1. Ravanelli, N. & Bongers, C. W. G. & Jay, O. (2019). The Biophysics of Human Heat Exchange. In J. D. Périard, S. Racinais (Ed.).  Heat Stress in Sport and Exercise (pp. 29-43). Switzerland, Springer Nature Switzerland AG Retrieved from https://doi.org/10.1007/978-3-319-93515-7_1 
  1. Gagnon, D. & Crandall, C. G. (2018). Sweating as a heat loss thermoeffector. Handbook of Clinical Neurology (v. 156) (pp. 211-232). Elsevier B. V. Retrieved from https://doi.org/10.1016/B978-0-444-63912-7.00013-8  
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