Optimizing Performance in the Heat I: Regulation, Acclimatization, and Pre-Cooling Strategies Explained
With summer on its way for the southern hemisphere, let's unpack some new research published around heat and athletic performance to help optimize performance this season.
Questions we will attempt to answer:
1.How does the body regulate temperature?
2.What heat adaptations occur in the body?
3.How do varying temperatures affect endurance performance?
4.Does pre-cooling improve endurance performance
5.Summary
1.How does the body regulate temperature?
In the brain is a structure called the hypothalamus where the anterior hypothalamus deals with increases in body temperature and the posterior hypothalamus deals with decreasing body temperature. The setpoint for temperature is around 37 degrees
Heat load
When the body is exposed to a heat load, receptors in the core and skin detect changes in the environmental temperature. Signs are then sent off to the anterior hypothalamus which responds with appropriate measures to maintain the 37-degree centigrade set point. This includes stimulation of sweat glands for increased evaporation heat loss, increased skin blood flow through withdrawal of vasomotor control resulting in vasodilation (becoming wider), and increased heat loss.
The body is around 20-30% efficient in converting energy into movement, with around 70-80% of total energy lost as heat
When core temperature returns to set point and/or skin receptors no longer are stimulated, signals for sweating and vasodilation are removed. This is an example of negative feedback
Calculating Heat Loss via evaporation
1000ml of sweat will result in around 580 kcal (or 1ml = 0.58 kcal) of heat loss during activities.
E.G. Tadej rides for an hour and burns 1200 kcal (remember 80% lost as heat, 20% converted to energy). 1200x .80 = 960 kcal as HEAT. 240 kcal remaining as muscle contraction.
To allow adequate cooling of the 960 kcal heat load, we divide that 960 by 0.58 kcal to arrive at our ml sweat amount needed to cool the body which is 1600ml. This assumes sweating is the primary method of heat loss. An interesting thought experiment nonetheless.
Cold Load
In contrast, cold exposure stimulates the posterior hypothalamus and the priority is to minimize heat loss and/or increase heat production. Again, we see that vasoconstriction (becoming narrower) of blood vessels helps to keep blood out of the periphery and minimize heat loss. If the core temperature continues to drop, shivering begins. Other reactions that occur are hair standing upright on the skin (pilomotor), increased thyroxine release to increase cell heat production as well and increased norepinephrine which also increases the cell's metabolic rate (Howley et al. 2003)
2.Heat Acclim