Heat, Humidity and Dehydration: Impacts on Performance

May 24, 2024

With warmer weather fast approaching across the board, let’s talk about how heat, humidity and dehydration impact our body’s ability to perform. To best understand how our bodies are impacted, we must first understand some physiological principles, starting with the heart.

Our heart pumps a given amount of blood per minute; this is called our cardiac output. Our cardiac output is made up of two parts:  1) heart rate (HR), the number of times our heart beats per minute and 2) stroke volume (SV), the amount of blood pushed out with each beat. So, Cardiac Output = SV x HR.

As we exercise, our heart rate begins to increase. This must occur to increase the blood supply to the working muscles to deliver oxygen and nutrients and remove waste. So right off the bat, we know just by exercising our heart rate increases.

At the same time, to keep our core temperature at a safe level, our bodies begin to send plasma, a component of our blood made up of mostly water, to the skin. One adaptation to regular endurance training is our bodies will begin this process earlier in exercise, so our ability to keep cool is improved and our perceived exertion of exercise stays at a lower level for longer. Since plasma is being sent to the skin, this reduces the amount of plasma (water) in the blood. As a result, our blood is more viscous and our heart rate must increase to continue to deliver blood to our working muscles at the same rate.

This is one reason why it is essential we do not allow ourselves to enter exercise in a dehydrated state and to do our best to stay hydrated while exercising. Couple a dehydrated state (an already reduced plasma volume) with a reduction in plasma volume due to our bodies cooling mechanisms and our heart will have to work even harder, by increasing our heart rate, to deliver the body what it needs.

So, at this point, we know our heart rate has increased due to exercise and we also know our body is sending plasma to the skin to keep our core temperature at a safe level. The longer we continue to exercise or the higher the exercise intensity becomes, the higher our heart rate will continue to rise. If we are dehydrated as we enter exercise and as we become dehydrated during exercise, our heart rate will continue to increase. You can see, there are many factors which can influence heart rate in and out of exercise.

Now remember, our cardiac output is also made up of our stroke volume, the amount of blood pumped with each beat. Another adaptation to exercise training is an improved elasticity of our left ventricle, this is the part of the heart which pumps blood to the body. As a result, the ventricle can fill more and pump more blood with each beat, so our stroke volume increases. With an increased stroke volume, our heart does not have to work as hard to deliver the same volume.

Through our training we are improving our heart’s ability to pump blood to the body without having to increase how hard our heart is working. This is why our resting heart rates are lower as endurance athletes. We can pump more blood with each beat without an increase in heart rate. You’ve got to love this stuff, our bodies are incredible!

Now, we are going to add heat and humidity into the mix. Training in the heat will cause our core temperature to elevate at a faster rate. So, our body’s internal cooling mechanism will begin to kick in earlier, resulting in reduced plasma volume and an increased heart rate as explained earlier. We will also begin to dehydrate at a faster rate and need to maintain our hydration levels much earlier in exercise. This is so easily forgotten in training and racing but is imperative for high performance and an enjoyable experience.

When we consider humidity, this has the greatest effect on the body’s cooling process.  When our body sends plasma to the skin, we sweat. For this cooling process to be effective, this sweat must evaporate from the skin for the heat energy to be dissipated and the body to cool. When the air is humid, this means the air is more dense with water. When the air is denser with water, this makes it more difficult for the water to evaporate from the skin. Our cooling mechanism is reduced. Also, as we sweat, we not only lose water, but essential electrolytes, which is why these must be replaced as we exercise.

With a reduced ability for the body to cool from the humidity coupled with an increased core body temperature and increased risk for dehydration from the heat, we are set up for reduced performance levels the higher the levels of heat and humidity get. When you enter a training or racing session at high levels of heat or humidity, it is essential to dial back the pace or power, right from the get-go. Depending on how high the heat and humidity, we cannot expect to maintain the same outputs as we typically would in more ideal conditions.

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