Heat Training – The Ultimate Guide
Updated: Jul 27, 2021
All runners push past their ‘comfort zone’ in training and even more so in racing, but many appear to get worried or even distressed about running in the heat. Yes, training in the heat is likely to mean that you do not hit your splits (especially if it is a longer workout) but there are many benefits to heat training and dealing with the heat.
The human body is a deeply complex organism and to ensure we function optimally, we aim to maintain a stable core internal temperature between 36.5-37.5° Celsius. However, humans are relatively inefficient and around 80% of the energy we produce is in the form of heat and when we run, we generate a greater amount of metabolic heat. This metabolic heat can increase the amount of heat produced by up to twelve times as much as at rest. Add in greater environmental heat levels and endurance performance can be sent spinning into a downward spiral.
Why does this happen? Well, firstly we must understand that the body aims to maintain its internal temperature in a process called thermoregulation which aims to maintain homeostasis and is regulated by a part of the brain called the hypothalamus. In order to understand thermoregulation we first must know how the body cools itself.
The body acts as a furnace, constantly producing metabolic heat and dispersing heat through various mechanisms, especially in hot temperatures. The body relies upon four cooling methods: conduction, convection, radiation and evaporation.
Radiation – This method accounts for approximately 60% of total body heat loss, making it the most effective form of cooling. Our body has the ability to send more blood to the surface of our skin via capillaries in a process called vasodilation.
Evaporation – The second principal source of heat loss is through the evaporation of sweat. This is more effective in low-humidity conditions because the air has a lower concentration of water molecules than the concentration of water molecules on the skin.
Conduction – This involves the loss of kinetic energy in the form of heat from the skin to the immediate surroundings. Different environments conduct heat away from the body at different rates, with water conducting heat away from the body 100 times more effectively than air.
Convection – Air very close to the skin is heated up and then as the air moves away from the body it carries heat with it. This is why we may feel cooler in windy conditions or when it’s hot, we use fan’s to cool us down.
These four methods are used by the body in order to cool us down when our core body temperature rises outside the normal range. The environment often dictates how effective the body is at being able to cool itself and return to homeostasis. The hotter or greater the humidity the slower the performance – Any one that completed the 2018 London Marathon will understand this.
What’s the ideal temperature to race in?
Unfortunately most of the research looking into the optimal temperature for human performance has been completed in labs using cyclists. However, recently, research has been published showing the impact of air temperature on marathon finishing times. The ideal temperature range for most groups of runners seems to be between 7° C and 15° C (44-59° F). Above and below this range, marathon times appear to get slower.
How much slower, it depends on the individual athlete and a few different environmental factors. A more humid environment reduces the cooling capacity of the body therefore limits athletic performance. The hotter the environmental temperature the slower the performance is likely to be. There is also a difference between how fast athletes are running. One research study showed that the slower the runner, the greater the impact of the heat. In temperatures up to 24° C running performance for elites was impacted by 2% (2-3 minutes over a marathon) as the runners got progressively slower, the impact of running in the heat became greater. Typically, larger athletes produce more metabolic heat therefore experience a greater percentage reduction in aerobic performance compared to smaller runners.
Why do we get slower in the heat?
The greater the rise in temperature and humidity the harder the body must work to cool itself rather than focus on performance.
Greater heat and/or humidity increases the physical stress placed upon the body and therefore increases the intensity of the run resulting in a higher heart rate compared to a lower temperature.
The increase in heat, stresses the cardiovascular system more than if the weather was cooler. The cardiovascular system (heart, lungs and blood supply network) is critical for the body to take in oxygen, transport it through the intricate network of arteries and capillaries and deliver this oxygen to working muscles. However, the same cardiovascular system is also important for cooling the body. If you remember that the most important method of cooling the body is radiation which uses that same capillary network that is vital to transport blood to muscles. But in hot conditions blood is diverted towards the skin (vasodilation) for cooling, limiting the supply of oxygenated blood to the working muscles.
The greater the heat the less effective the body’s cooling mechanisms become. It is very difficult to lose heat from the body when the skin and air temperature are the same or very similar. Likewise, when the humidity level is very high (a high concentration of water vapour is in the air) it makes it harder to sweat and therefore lose heat.
Running in the heat does have its advantages!
The impact of heat on athletic performance has been the focus of scientific study for decades and heat acclimation produces a number of biological changes that help improve aerobic athletic performance. So what are the physiological changes that result from heat training?
Increase in Sweat Rate – You begin to sweat more at lower body temperatures to begin the cooling process earlier.
Reduced concentration of electrolytes in the sweat – Your sweat contains less salt helping to improve the balance of electrolytes. Maintaining the optimum balance of electrolytes is vital for all cells in the body in-particular muscle cells used for running.
Lower skin and core temperatures – The body learns how to regulate its core temperature more effectively.
Lower heart rate – Running in the heat increases the stress placed upon the body and heat acclimation helps to lower heart rate. It’s hypothesised this is due to increased plasma volume (more on this in a minute) which improves cardiac filling.
Allow us to become more comfortable running in the heat – This may not be a physical change but if we do something repeatedly we normally become accustomed to it and running in the heat is no different.
Increased plasma volume – Training in the heat increases our plasma volume and maximal cardiac output (the maximal amount of blood we can push out of the heart in one minute and therefore delivers more oxygen to working muscles. With a greater plasma volume, you are able to send blood to the surface of the skin for cooling purposes bit still adequately supplying the muscles for performance.
Does Heat Acclimation have Benefits in Cooler Temperatures?
If you have an upcoming race in the heat, training in the heat makes sense. Whilst this is true and heat acclimation improves performance in hot conditions, heat acclimation has also been shown to improve performance in cooler temperatures. In the same way that many Olympic athletes train at altitude and race at sea level to improve performance the same is true for heat acclimation. In fact research has shown that heat acclimation may actually provide more substantial improvements in aerobic athletic performance than altitude acclimation. Some of the benefits include reduced oxygen uptake and blood lactate at a given running intensity, improved skeletal muscle force generation, greater blood plasma volume, muscle glycogen sparing and improved myocardial efficiency.