Heat adversely affects the body’s ability to do work, but strategic utilization of heat in a training regimen can facilitate performance increases. Dr. Chris Minson, one of the leading experts and researchers on this topic, explains how heat training can improve your performance and offers tips on how to prepare for events in hot conditions.

For more information on heat training and endurance performance, check out Science of Getting Faster Ep 1.

Why Does Heat Reduce Endurance Performance?

A little heat isn’t a bad thing. Small amounts of heat within a muscle actually improves muscle performance, which is why warmups are vital. However, too much heat is detrimental to performance, especially in endurance sports. The longer you go, the more heat becomes a factor. The reason is that your muscles begin to compete for blood flow with the body’s thermoregulatory process.

Anytime you exercise, only about 25% of the energy you expend is turned into actual work. The other 75% is converted to heat. This heat increases the body’s temperature. As your core temperature rises, the body begins to prioritize cooling instead of oxygen delivery to the working muscles, which leads to a decrease in performance.

How Does the Body Thermoregulate?

We use our blood to dissipate heat. Blood that flows through the muscles gets carried to the core and finally out to the skin, where we have an interface between our body and the environment. Finally, we have the sweat mechanism. The more blood flow to your skin, the more you’ll sweat. As your sweat evaporates, it cools the skin and reduces the temperature of the blood. The body is remarkable when it comes to handling heat. However, dealing with humidity is difficult for our bodies due to the lack of evaporation.

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Even if you are training in a cool room, riding without a fan will create a humidity bubble around you, and the sweat that drips off of you does little to cool you down. To help increase your performance in the workout, you can do two things. First, increase the airflow with a good blower fan. The second thing is to reduce the humidity in the room with air conditioning or a dehumidifier.

Heat Acclimation Study

In 2010, Dr. Minson published his study; Heat Acclimation Improves Exercise Performance. The idea for the study began while Dr. Minson was helping a marathon runner prepare for the hot conditions of the 2008 Beijing Olympics. He wanted to ensure that heat acclimation wouldn’t decrease performance. Dr. Minson hypothesized three outcomes to heat acclimation. First, would there be increased performance in normal conditions? He didn’t think there would be. Second, would performance increase in hot conditions? Third, would there be a decrease in performance in cool weather?

Design

Dr. Minson designed the study around cyclists partly because it’s easy to quantify the workload that they’re doing. This not only helps reduce variables but helps to record good physiological data in the lab. The cyclists were a mix of well-trained men and women who were mostly category 1 and 2 racers. All of the men had a VO2 Max above 65 mL/kg/min, and the women ranged from 58-65 mL/kg/min.

The cyclists were randomly assigned into two groups and faced a battery of tests before and after the intervention. The intervention spanned ten days over two weeks with a break for the weekend. The cyclists rode at 50% of VO2 Max for 45 minutes, took a ten-minute rest, and then rode for another 45 minutes. The intervention group did this regimen at 104 degrees Fahrenheit, while the control group completed the same rides but in a controlled 55-degree room. Both groups used fans.

A key to the design was to separate the additional heat from the increased intensity it causes. Naturally, the intervention group would have higher heart rates than the control group, leading to training adaptation. To help mitigate the effect, Dr. Minson chose low-intensity rides for all participants.

Testing

To get an overall picture of performance, Dr. Minson looked at VO2 Max and lactate threshold. At the time of the study, many researchers weren’t using FTP, Critical Power, or other measurements. He also measured skin blood flow and other temperature regulation markers but did not include those in the paper.

There were a significant number of tests, both before and after the intervention. The researchers completed the tests in both cool and hot environments. The three primary tests were a VO2 Max test, a modified lactate threshold test, and a one-hour time trial.

Results

Dr. Minson observed two key outcomes in the results. The first, which he already suspected, was that he saw real performance improvement in hot conditions. This meant that the heat acclimation worked very well. The second result was a bit surprising. The intervention group saw VO2 Max and lactate threshold increases in the cooler conditions—a 2-8% increase from almost every athlete in the group.

Dr. Minson believes that some of the increased performance was due to the training effect caused by elevated heart rate, which was one of the early criticisms he received. He admits that he knew that might be the case. But, he says that if you do heat acclimation right, “there’s always a benefit in the heat. For some athletes, adding heat, whether it’s physiological or a different training stimulus, there is some training and performance benefit.”

Takeaways for Your Training

Dr. Minson recommends a few things if you want to use heat training to increase your performance. If you’re preparing for an event that will take place in hot conditions, don’t wait until the last minute to start acclimating. The earlier you start, the better the results. If possible, try to mimic the race environment as close as possible, especially regarding the humidity.

Another way to get in some heat training throughout the year is to add heat to a couple of aerobic rides each week. This can be as simple as training in a hot room or turning the fans off. How hot should the room be? For the sake of simplicity, Dr. Minson recommends using a scale from zero to ten, with zero being entirely comfortable and ten being the hottest and most miserable you can be. You want to be around a seven on that scale.

First and foremost, you want to make sure to add heat training safely. When you start to incorporate heat training, do so carefully and make sure to listen to your body. Heat training can be a massive strain, especially if you don’t hydrate and fuel well. Start very slow and only complete easy endurance workouts. There’s little to be gained from attempting a difficult workout in the high heat and the risk of heatstroke is life-threatening.

Researcher and Background

Dr. Chris Minson has been performing research on human adaptations to environmental extremes, biomarkers of cardiovascular health, and autonomic-vascular regulation for over 20 years at the University of Oregon. You can learn more about Dr. Minson’s work at the Human Cardiovascular Control Lab.