It’s widely accepted as fact that racing in the heat decreases aerobic performance and can be amplified by high humidity. Your body’s heat production is directly related to the effort you are outputting.
This is increased dramatically by lack of efficiency of humans while exercising. For every 100 watts you are producing you are creating 400 watts of heat! This means for some more powerful riders who are going along at 250 watts they are producing 1000 watts of total energy that needs cooled. Your body has a variety of ways to dissipate this heat and sweating is often the most important and efficient one. Sweating will allow you to disperse heat but in longer endurance events will likely lead to dehydration, which makes it more difficult to regulate body temperature. Things can get much worse when the environment is both hot and humid as it becomes even more challenging to remove this heat and you have to rely solely on sweating.
For sometime it was thought the brain was controlling the shut down of the body because if it reached 103 degrees performance would decrease, much like an emergency brake preventing you from injuring yourself. However, they have since documented runners with internal temps in the 105 degree range during competition with no issues in performance.
So what is causing the problems and why is racing in the heat so hard? It come’s down to a cascade of problems coming from dehydration and the body’s lacking ability to efficiently cool itself.
As you become dehydrated and overheated plasma volume are reduced, whilst blood vessels expand, and blood begins to pool near the skin. This makes keeping both blood pressure and blood volume up hard and results in the heart working much harder to pick up the slack. Most people will see a 5-10% decrease in blood volume which dramatically compromises the delivery of oxygen and components for metabolism and waste removal.
While heat can improve the capacity of muscles contraction (overall strength) it has a detrimental effect on time to exhaustion. More often than not, muscular exhaustion in the heat occurs from dehydration long before lack of energy. However, lack of proper glycogen replacement can cause both problems to occur especially in longer events (3+ hours) as you continue to push forward.
In endurance events that require long steady efforts, as body temperature rises your body displaces blood to your skin to be cooled and reduces cardiac filling, resulting in a reduced maximal cardiac output. These changes to lung and heart function are most notable to those with a lower VO2max. The reduction results in a shrinking cardiovascular reserve, which is the primary limiting factor for aerobic exercise performance. This creates a far greater sensation of effort at a lower power output.
There are a large variety of effects that can be linked to the noticeable sense of effort but early fatigue or slowing of pace can be explained by greater cardiovascular strain, elevated relative exercise intensity, increased perceived exertion, and associated behavioral changes toward the perceived exertion.
Last but certainly not least, there is increased discomfort and affects on pain tolerance, mood, and motivation, all of which can influence performance. Heat stress directly increases an individuals RPE for an effort that could have been tolerated in more moderate temperatures.
Explaining why performance is decreased in the heat, predicting how much performance will be affected, and how much certain levels of dehydration will influence this, is quite complex. It is clear that there is not one factor but many factors that work together against you.
From a practical point of view: we can’t usually change the environment, therefore it is recommended to minimize dehydration and look for opportunities to cool skin and body. Still looking for help? We offer cycling and running coaching programs that can help get you on the path to being able to handle the heat better with a personal plan!