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Heat and Heart Rate Zones: How Weather Can Affect Your Training HR

We explain why the heat and humidity can affect your training zones, and what to do about it.

Heart rate training is a popular approach to endurance training, both for runners and for other endurance athletes like cyclists.

Heart rate training involves monitoring your heart rate during exercise and aiming to keep it within specific predetermined “heart rate zones,“ depending on your age, fitness level, and workout goals.

However, multiple factors can affect your heart rate and heart rate zone ranges, such as stress, caffeine, altitude, hydration, and the weather.

In this guide, we will discuss heart rate training, specifically how heat and heart rate are related and how you may need to adjust your training when that air temperature rises.

A person checking their running heart rate in the heat.

What Is Heart Rate Training? 

Heart rate training uses your heart rate, or beats per minute (bpm), to monitor your effort while running instead of using pace, power, or RPE (rate of perceived exertion).

This can be an effective training method for helping runners stick to certain intensities and effort levels during physical activity without overdoing it.

Brett Lato, the Manager of Training and Education at Polar, one of the leading companies at the forefront of heart rate monitoring technology for athletes, says that heart rate zone training can be an effective tool for runners at every fitness level. 

“Essentially, each heart rate zone represents a different level of intensity so runners can better understand how hard they are pushing themselves during a training session,” explains Lato.

“Precise knowledge of your training intensities can ensure you receive the desired effect from your sessions,” Lato says many factors impact your heart rate, including your fitness level and aerobic capacity. 

“Because your heart rate is personal, it’s important to determine your unique maximum heart rate to optimize your training zones,” advises Lato.

“Using a heart rate monitor is the most effective way to determine your maximum heart rate and personalized heart rate zones, so you can accurately track your progress over time.”

A person running with a heart rate monitor.

Heat and Heart Rate: Can Heat Make Your Heart Rate Go Up?

One of the downsides of heart rate training is that even for the same individual, heart rate during exercise or even during everyday life can be impacted by factors such as stress level, hydration level, and even the hot weather.

This means that other factors, in addition to the workout intensity or type of exercise, can impact your heart rate while running or working out.

In particular, running in the extreme heat and humidity can increase your heart rate and therefore decrease your performance.1Maughan, R. J. (2010). Distance running in hot environments: a thermal challenge to the elite runner. Scandinavian Journal of Medicine & Science in Sports20(3), 95–102. https://doi.org/10.1111/j.1600-0838.2010.01214.x

According to the Journal of Science and Medicine in Sport, marathon performance declines by around 0.5% for every 1°C increase in temperature above 10°C (50°F).

Humidity makes running in high temperatures even more taxing on the body because when the moisture content of the air is higher, it feels hotter.

“Running in extreme temperatures can challenge anyone, even elite athletes,” says Lato. “It’s important to make adjustments and tune into what your body needs to prevent risk, especially in high heat.”

Lato further explained how temperature affects your cardiovascular system:

“Exercise generates heat and increases our body temperature,” says Lato. “As your body temperature increases, whether from exercise, heat or a combination of both, your heart must work harder to increase circulation in an effort to cool down.”

So, when exercising in an already hot environment, your body’s core temperature can increase rapidly and exceed the “safe“ range.

As such, the body doubles down on the thermoregulatory mechanisms normally initiated during exercise to cool your body down: sweating and increased blood flow to the skin.

For sweat to evaporate effectively, the air needs to be relatively dry. Therefore, in high humidity, when the air is saturated with moisture, evaporation can slow.

These are both energy-intensive processes, so your heart rate can increase.

This explains why your heart rate running might be higher in the heat and humidity,2González-Alonso, J., Crandall, C. G., & Johnson, J. M. (2008). The Cardiovascular Challenge of Exercising in the Heat. The Journal of Physiology586(1), 45–53. https://doi.org/10.1113/jphysiol.2007.142158, but just noting a higher heart rate during summer workouts or running in the heat isn’t necessarily enough.

A person bent over, looking at their running watch.

“If you get too hot while exercising, your body temperature can rise above normal, leading to heat exhaustion and other heat-related illnesses, like heat stroke,” warns Lato.

“You’re also more susceptible to dehydration, so it’s important to drink water to help control body temperature.”

Lato provides some advice for running in the heat:

“Be sure to pay attention to warning signs of dehydration and heat exhaustion when running in heat, like dizziness, nausea, and headaches.

In addition, muscle cramps, extreme thirst, dry mouth, and low blood pressure are also signs of dehydration.

As for heat exhaustion, you may also experience excessive sweating.

Your heart rate is another effective way to listen to your body during your training, especially in the heat to make necessary adjustments and continue running safely.”

In other words, even though you might be frustrated to see a higher HR on your HR monitor when running in the heat, you should use that biometric data to provide insight that your body is working overtime.

You should look at the heat index using a heat index calculator like the one here. This can help you determine the “real feel” of the heat and the impact of heat on heart rate.

According to the Road Runners Club of America (RRCA), you should avoid running outside if the heat is above 98.6 degrees Fahrenheit and the humidity is above 70-80%.

If you are experiencing a heat wave or the conditions are simply unsafe, running on a treadmill in the air conditioning may be your best bet.

A person looking at their fitness tracker.

Do You Need to Adjust Your Heart Rate Zones In the Heat?

“Because your heart rate will be elevated in the heat, you’ll find that you’re reaching higher zones with a slower pace, and that’s okay. ”

“When training in the heat, you may want to reduce your average speed rather than trying to maintain your regular pace at a higher heart rate,” advises Lato. “This can be done by slowing down or extending recovery periods during interval runs.”

While some runners may be concerned that slowing down will negate the purpose of HR training for runners, Lato says this isn’t true.

Rather, adjusting your HR zones in the heat is exactly what heart rate training is about—using your heart rate to guide your training.

“You’ll find that even with this adjustment, you can still see benefits from following your heart rate zone training while reducing risk,” suggests Lato.

A person tying their shoe getting ready to run.

Do My HR Zones Change In the Cold?

Generally, it’s thought that the main impact of temperature on heart rate zones or heart rate training comes on a hot day or a humid day, not on a cold day.

“In cold weather, it’s possible your heart rate will slow. This means you may be able to maintain a faster pace or higher intensity at a lower heart rate,” explains Lato.

“Your heart rate zones will not change, but training in cold weather is a great time to improve your strength and endurance since you’ll be less fatigued.”

In other words, you might have to push your body a little harder to hit the same heart rate zones in the cold, but this is where you “win back” the adjustments in the effort you had to make with HR training in the summer heat.

Does Dehydration Affect HR Training?

Some studies suggest that it’s actually dehydration—and the subsequent drop in plasma volume—that causes the impact of heat and heart rate we generally see (an increase in HR exercising in the heat), rather than the heat itself.

A 1.5% weight loss due to dehydration can increase your heart rate by about 7 bpm, which is significant enough to impact your heart rate zones.

According to the American Heart Association,3Staying Hydrated – Staying Healthy. (2014). Www.heart.org. https://www.heart.org/en/healthy-living/fitness/fitness-basics/staying-hydrated-staying-healthy the best you can do to keep your fluid levels up while running in the heat will help minimize the heart rate elevation in the heat as the temperature rises.

An increased heart rate can also be a sign of dehydration. 

Lato says, “If you notice your heart rate increasing with no change in your pace or other variables, you should consider resting and increasing your fluid intake.”

Here again, using a heart rate monitor with your training can help guide your workouts and give you insights into your fitness.

“A heart rate monitor is the most effective way to measure your intensity during training and track your progress over time.”

If you are interested in using the heart rate training method, the first step is to accurately calculate your heart rate zones. Learn how to calculate your heart rate zones.

If you find your heart rate is unusually high during training and your day-to-day activities, check with your healthcare provider to ensure you don’t have any preexisting heart condition and your heart health is in tip-top shape.

References

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Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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