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Training in the Heat; Why You Feel Like Crap, But Why It May Be Good for You

We all know how we feel when the heat index is over 100. Somehow we find ourselves on the blacktop, not even a tree branch overhead with the sun beating down on us. It feels like an Earth killing sunspot just arrived, just now during your threshold interval, you are not going to make it to water in time and you can feel the tar in the road under your tires is getting squishy. What you know is that you feel tired, sluggish, and above all hot. What you may not know is that the effects of getting acclimated to the heat may help your performance later on.
wunderground hot texas
First, we can look over the negative effects of this heat on your cycling performance. The negative effects of heat are very well documented. It’s pretty easy to control and study the effects of heat on athletes, especially cyclists. According to the study “Influence of Body Temperature on the Development of Fatigue During Prolonged Exercise in Heat” participant’s body temperatures were controlled by immersion in a water tank, then during a bout of cycling exercise, the test subject’s core temperature, blood flow, and skin temperatures were recorded along with the cycling metrics during their time trials. The cyclists with the higher core temperatures performed worse across all tests, the time trial, rate of perceived exertion, and time to fatigue. Why the cyclists with controlled higher body temperatures performed worse is contributed most to a few physiological factors that happen to your body in heat and with dehydration.
Your brain can hinder your own performance. Your core temperature is regulated by your Central Nervous System or CNS. Your body feeds information from your core, muscles, and organs back to the Central Nervous system and your Hypothalamus and your body tries to maintain a healthy temperature.  You start to sweat but also your Brain and CNS can hinder your ability to perform the effort.
The impairment of heat on the human body during exercise is further seen in the review “Mechanisms of Aerobic Performance Impairment from Heat stress and Dehydration” the degradation of performance in heat is caused by adjustments your body makes to account for high skin and core temperatures, and where you need to sweat more. When you start to overheat your body increases its blood flow to the hot areas. Since your sweat comes from your blood plasma, sweating more lowers your blood volume. On top of this lowered blood volume, you body’s metabolism is higher, and your hemoglobin has a harder time carrying oxygen to your muscles. All of these thing hinder your performance in the heat weather it’s an easy endurance ride or a race. On top of these negative factors, something that multiple studies allude to but have a hard time measuring is that RPE or rate of perceived exertion is higher. When it’s hot outside and you push yourself it just feels harder. Much of this is also contributed to your CNS telling you to shut it down before your hurt yourself. In these cases we are not even getting into the true effects of dehydration which also increases the above negative performance factors but further changes your muscles ability to contract.
airplane sweat
Image brought to you by “Airplane” the movie
All of this seems terrible. You can’t even get the full workout done outside so you may as well head to your trainer or the local training center and get your prescribed interval workout in right? Well, maybe some of the time BUT there is something to be gained in acclimatizing to the heat.
There is one very influential study “Heat Acclimatization Improves Exercise Performance” by Santiago Lorenzo that I encourage every cyclist to read. The study took trained cyclists, put them through a 10 day heat acclimatization program where they trained in the heat, cool, or normal temperatures. In the end the heat acclimatization group had an 8% higher Vo2 max and a 5% raise in Time trial performance and 5% higher lactate threshold. What created these gains is what we want to look at specifically to gain an advantage not only in the heat but also if you train in the heat and race in cooler temperatures.
The increase ability in the heat trained athletes had certain measured physical adaptations. The heat trained athletes had blood plasma expansion, improved myocardial efficiency, and better ventricular compliance. Basically you have more blood plasma from training in the heat which increases your heart’s ability to move blood and oxygen and increases your Vo2 Max and performance.
heat 1
This graph shows the cyclists before heat training in Black and post heat acclimatization in white showing a higher Vo2 max, higher work in Kilojoules performed in a time trial, and a higher lactate threshold in the heat acclimated group.
heat 2
Similarly in these charts the heat acclimatized group shows a higher cardiac output and stroke volume. The Heart rate graph is interesting as the PRE acclimatized group shows a lower heart rate seeming like a raised efficiency, less cardiac output needed for a given intensity but in the heat, the experimental heat acclimatized group shows a higher HR. Since Cardiac output=Heart rate X Stroke Volume, the heat trained group has a higher cardiac output overall. The ability to maintain a higher heart rate in the heat trained group may be a central nervous system adaptation.
In review. Training in the heat is hard but if you stay hydrated you will acclimatize to the heat and gain blood volume and raise cardiac output. Everyone is different but once you acclimate to the heat you stand to gain a higher VO2 max, power at Lactate threshold and will be all around faster on the bike.
Author: Zack Allison – Source Endurance


Jose Gonzales – Alonso, C. P. (1999). Influence of body temp on the development of fatigue during exercise in heat.pdf. J,app.
Samuel A Cheuvront, R. W. (n.d.). Mechanisms of Aerobic Performance Impairment from Heat stress and Dehydration. US Army Research Institute of Enviromental medicine.
Santiago Lorenzo, J. R. (2010). Heat Acclimatization Improves Exercise Performance. Journal of Applied Physiology Published 1 October 2010 Vol. 109 no. 4, 1140-1147 DOI: 10.1152/japplphysiol.00495.2010.