A Reminder About Running in Heat and Humidity

Just a reminder for runners trying to perform in the heat and humidity. Expect your paces to slow down, you have heard this before, but it is good to hear it again.

Sweat helps us stay cool through evaporation. When it is humid, your sweat does not evaporate as easily, keeping your core body temp higher. This adds more stress, and in turn, increases heart rate and perceived exertion. You will adapt, but you need to be ok with slowing down….remember you are still working just as hard.

If you are still worried, make a trip up the the mountains or a dry environment and see for yourself how fast you really are and how good you really feel:)

Also check out this HEAT RUNNING CALCULATOR which gives you an estimate of how much your pace changes from heat.  I don’t think it takes humidity in consideration, so that is even another added factor.

Here are some other considerations from running websites:

More Stress on the Heart

Ideally, during exercise we want the heart to pump out as much blood as possible in each beat so that the heart rate doesn’t skyrocket to sustain a given workload. What happens in the heat, however, is less than ideal.

With more blood at the periphery, there is less blood flowing to the heart. This decreases cardiac filling and stroke volume — the amount of blood that is pumped from the heart. To compensate, heart rate increases to sustain the workload. As a result, the relative intensity of exercise increases, more stress is placed on the heart and we max out sooner. In other words, an 8-minute pace may feel like a 6-minute pace because the heart is working that much harder.

Also, if blood volume decreases from high sweat rates (a loss of plasma) there is an increase in blood viscosity — a higher concentration of red blood cells — which puts more stress on the heart and vessels. ~competitor

Working Muscles Suffer And Anaerobic Modes Kick In

When blood volume is split among competing interests during exercise in heat, the next victim is active muscle.

Muscles engaged in activity suffer because they aren’t getting as much oxygen from the blood. For endurance athletes oxygen is gold; it’s the fuel that allows us to sustain exercise for longer durations, and without it we’re forced to rely more on pain-inducing anaerobic (without oxygen) modes of producing energy.

Increased anaerobic energy production affects exercise at all intensities and causes a slew of issues including higher total energy expenditure and blood lactate accumulation. Also, carbohydrates are used for energy more than lipids (fat), and since carbohydrate fuel stores are extremely limited in the body, exhaustion is reached much sooner.

In the end, this shift from aerobic to anaerobic modes will generally result in a faster onset of muscular fatigue. ~ competitor

Thanks for reading.

Tom Clifford
USATF Level 2 Endurance Coach
Founder of Without Limits
Race Director

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