Training in Heat: How It Affects Heart Rate and Performance

Summary: Should you train when it's hottest, or is it better to move your session? This article explores how heat affects performance by requiring blood to supply both muscles and cooling through the skin. The main question is why the heart rate increases and exertion feels more intense in the heat, even when the intensity remains the same. By linking heart stroke volume and pulse with the body's cooling needs, it becomes clear why some sessions get tougher as the temperature rises. The conclusion is that you can still achieve quality training in the heat but often need to plan smarter, hydrate properly, and sometimes choose a cooler time to get the most out of your sessions.

Introduction

What's the deal? Is it a good idea to head out for a workout on a hot sunny day in the middle of summer? Maybe it's smarter to take a rest day or adjust the training schedule a bit! A fun fact is that in Sweden, the hottest day of the year has often occurred in early or mid-August (August 4, 2014, August 3, 2013, and August 19, 2012). A period when many competitions take place here in Sweden.

One heart – one circulatory system

Blood is constantly being pumped around the body thanks to our dear heart. When we exercise, the demand for oxygen and energy to the muscles increases, requiring the heart to pump more blood to them. It's pretty simple. But when the sun is blazing and the heat increases, the need for cooling also rises, which is one of the many missions of our circulatory system. More of the hot blood is directed to the skin so that heat can evaporate. This, along with sweat forming a cooling film on the skin, makes our cooling system work very well. But it affects other systems in the body whether we like it or not. After all, it's just one interconnected system.

The biggest limitation is that we always have a practically limited amount of blood in our bodies (excluding blood donors and those who are blood doped) along with a single heart that pumps it around. The blood is almost always redirected to the area where it is most needed at the moment. If your fingers get cold when you eat, it's because you have more blood around the body's core. But when you start running, it doesn't take more than a few minutes before most blood vessels in your legs dilate, and blood is sent there to handle the increased need for oxygen and energy. And of course, if you're hot, more blood moves toward the skin for heat evaporation. When the blood is diverted away from the muscles and our internal organs, some effects arise that affect performance:

• Less oxygen and energy delivery to the muscles.
• Reduced removal of waste products from the muscles after contraction.
• Poorer lactate uptake in the liver as less blood is directed to internal organs. This consequently leads to poorer lactic acid management.
• Your central fatigue increases, warmer core temperature combined with a warmer brain (both due to poorer blood flow to the organs) leads to a more general feeling of fatigue.

Heart Rate and Exertion

We have our main pump, the heart, which can increase blood circulation to meet the increased need for blood in different parts of the body. The heart's ability to pump out more blood is primarily increased through two mechanisms:

1. More blood with each beat (stroke volume).
2. More beats per minute (stroke frequency, or heart rate in everyday language).

When you train, your heart becomes stronger and pumps out more blood with each beat, meaning that your heart rate doesn't need to increase as much at a given workload. If you take the textbook example of a stroke volume of 70 ml blood/beat multiplied by the heart rate (e.g., 60 beats/minute), you get the cardiac output (4.2 liters/minute). Higher cardiac output is linked to higher maximal oxygen uptake (VO2max) (1). Heart rate is also directly linked to perceived exertion, so with a stronger heart (higher stroke volume) you can train harder with lower perceived exertion than someone with lower stroke volume who must have a higher heart rate for the same workload. Your resting heart rate also decreases because the heart is stronger/larger and pumps more blood per beat even at rest — which is clearly positive for health and longevity. Genetic variations naturally exist; some are born with larger and stronger hearts and have a lower resting heart rate, while others are born with a smaller heart volume. In any case, the heart is a very trainable muscle and can significantly increase in volume if you train it.

The Impact of Heat on Heart Rate and Stroke Volume

We have a set amount of blood and just one heart. When we need cooling down, more blood is diverted toward the skin — heart rate increases — and it becomes more challenging. The tricky part is that heat has the most impact when the intensity rises, and muscles and skin really have to compete for the available blood. Similarly, a hypothesis for why we get stitches when training with food in the stomach is that the digestive system and muscles fight over the blood supply.

In a controlled study from 2010, a group was asked to cycle for 60 minutes at 57% of their max heart rate. During the test, it was 23 degrees and 35% humidity (a typical Swedish summer day). Half of the group cycled in warm clothes and the other half in regular thin workout clothes. Compared to those who cycled in regular clothes, the stroke volume decreased by 9% and the heart rate increased by 7% (compensation), while the other half, who cycled in a vinyl jacket and long pants, experienced a 14% lower stroke volume along with a significant heart rate increase. Generally, those who cycled in these clothes reported higher perceived exertion and also sweat considerably more. Interestingly, the stroke volume wasn't directly affected by the heat during the milder exposure, but the heart rate increased significantly, which in turn increased the perceived exertion.

Practical Tips

Surprisingly, there is little research in this area. Most of the information for this article is taken from the book "Exercise Physiology" by McArdle & Katch. Generally, we're a bit skeptical of information from books since practically anyone can write and publish a book, but getting a peer-reviewed, scientific article usually requires stricter evaluations than publishing a book. 🙂 Nevertheless, the book has references, and the information is well-supported, so here are some basic tips from the book and a couple of studies:

• At temperatures above 26 degrees, you need to be meticulous with water and salt intake. Read more in our article on salt replacement for tips.
• Above 29 degrees, training in direct sunlight should be avoided.
• If the temperature exceeds 31 degrees, the recommendation is to avoid training at that time and schedule it for a cooler time of day.

Good to Know

• The optimal temperature for performing in a marathon is 10–12 degrees. Some studies suggest that for elite runners, the marathon temperature should be around 4–10 degrees due to high intensity and duration.
• The majority of negative effects from training in heat can be counteracted with sufficient intake of water and salts, along with the occasional cold shower on the head.
• It's not directly negative to train in the heat — you'll still gain training effects. But if your training time is limited, it may be wise to use it smartly: avoid long sessions in direct sunlight and 30+ degree heat, opting instead for intense intervals on a treadmill in an air-conditioned gym that day.
• Last but not least — don't carry around more insulation (body fat) than necessary if you want to perform at your best. Besides, a heavier body with more dead weight is more energy-demanding to move, and fat insulates the body's heat, slowing down natural cooling.

This is how heat is released from the body

Heat is primarily transported by the blood and released in four main ways:

• Radiation – heat radiation (infrared radiation) occurs through the skin. If you've ever seen an action movie where they use infrared goggles at night, it's heat radiation you see as red, pink, and yellow fields.

• Conduction – heat energy transferred between molecules. Warm air moves towards cold or warm air that migrates towards water and heats it. Think of a hydronic heating system.

• Evaporation – sweat sits on the skin, gets warmed up, and evaporates. This cooling process is energy-intensive and highly dependent on the environment. If it's high humidity outside, sweat doesn't evaporate, which hinders cooling (if there's more water in the air than on the skin, the sweat simply can't evaporate).

• Convection – air and gas currents, like wind, cool the skin and carry away heat. More wind cools more.