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 Acclimatization Adaptations
We will unpack heat acclimatization more in the next section by looking at recent research in the space of heat and cycling. Before that, what are adaptations that occur with exposure to heat?
While the exact temperature of what constitutes "heat" is different for everyone, in general when the body experiences a disturbance in homeostasis it is different depending the the level of exposure.
Primary adaptations are an increase in plasma volume (fluid component of blood), earlier onset of sweating, higher sweat rate, reduced skin blood flow, and more shock proteins in the blood.
Plasma volume increases by around 10-12% (Brooks et al 1987) which helps to maintain the output of stroke volume and sweating during heat and respond to smaller temperature changes.
Stress proteins called "shock proteins" are also produced when the body is exposed to heat and their function is to protect cells from heat injury by stabilizing and refolding damaged proteins that result from thermal overload.
The sweating rate increases by around a threefold increase compared with prior adaptation.
Finally, an increase in aldosterone helps to reduce sodium and chloride losses by the kidneys. (Wydham 1973)
Primary Adaptations Resulting from Heat Acclimatization |
Increased plasma volume |
Earlier onset of sweating |
Higher Sweat Rate |
Reduced sodium and chloride loss in sweat |
Reduced skin blood flow |
Increased heat shock proteins in tissues |
3.How do varying temperatures affect endurance performance?
In a study published in 2022, Valenzuela et al approached this question with some interesting findings
They had data from 48 male pro road riders as well as 26 pro female road riders looking at 5s, the 30s, 5min, and 20min mean maximal power data.
Then they looked at those powers mentioned previously at
<0 Degrees Centigrade
0-5 Degrees Centigrade
5-10 Degrees Centigrade
10-15 Degrees Centigrade
15-20 Degrees Centigrade
20-25 Degrees Centigrade
25-30 Degrees Centigrade
30-35 Degrees Centigrade
>35 Degrees Centigrade
Overall Averages | Performance loss |
<0 °C | |
0-5 °C | 9% |
5-10°C | 2% |
10-15°C | |
15-20°C | |
20-25°C | |
25-30°C | |
30-35°C | 7% |
>35°C | 18% |
Key findings
Males
Mean maximal powers were decreased below 10 degrees centigrade and over 30 degrees centigrade.
Females
5s and 30s were only impaired below 5 degrees centigrade, while long efforts of 5min and 20mins decreased above 25 degrees centigrade.
The overall optimal temperature is between 10 degrees centigrade and 30 degrees centigrade. Females can tolerate slightly lower temperatures while males tolerate higher temperatures better before performance loss.
What if we utilized a pre-cooling jacket to regulate core temperature, potentially improving performance, given the adverse effects of high temperatures on male and female riders at varying degrees?
4.Using pre-cooling to improve endurance performance
Faulkner et al. 2019 looked at using pre-cooling and its impact on performance
Results below:
26 Trained Cyclists Completed a time trial in two conditions where they needed to achieve 1000 kilojoules work. | Pre-cooling (using a cooling vest and sleeves) for 30mins plus 9 minutes warm up before time trial effort. 5mins between taking off cooling vest and start of time trial | Control (no pre-cooling) |
24 °C | -42s (-1%) | 61mins |
27 °C | -1min35secs (-3%)* | 63mins |
35 °C | -4mins (-6%)* | 69mins |
The above data suggests that using a pre-cooling vest can have significant* benefits the hotter the environmental temperatures become and can be used as a performance-enhancing strategy.
Summary
The hypothalamus regulates heat and cold responses in the body
The anterior hypothalamus regulates heat response
The posterior hypothalamus regulates cold response
The set point of temperature in the hypothalamus is 37 degrees centigrade
1ml of sweat loss is equivalent to 0.58kcal of energy
Optimal temperatures for road racing are between 10-30 degrees centigrade
Pre-cooling vests have a significant performance-enhancing effect for temperatures above 27 degrees centigrade
Next week, we will delve into further research on heat and thermal exposure!
Train Hard and Prosper!
Darrin Jordaan
MSc (Med) Biokinetics WITS
HMS (Hons) Sports Science UP
BK 0016934
CSCS
UCI Level 1 Cycle Coach
IronMan certified coach
