Burning with desire: training your body to perform in the heat

This article:

• Explains the relationship between heat and running performance
• Shows how runners can harness the principles of heat training to improve running performance in cooler weather too

 

Most endurance athletes hate the heat, particularly those who live in temperate climates, where even in midsummer it’s often possible to dodge the worst of it by training in the morning, before the day has worked up its full onslaught. But then we encounter a race in which we can’t hide from the sun. We struggle through, grumbling and hoping to avoid the medical tent, swearing one more time about how much we hate heat.

Sounds familiar? One of my favorite coaching memories involves a talented woman who early in our association informed me she was terrible in heat. “You’ll have to prove it,” I said, “because I doubt you’ve ever really trained for it.” Two months later she ran a personal best 5K of 31 minutes.

The fact is it IS possible to compete in heat - if you are properly prepared. In the 1996 Atlanta Olympics, Portuguese distance star Maria Fernanda Moreira Ribeiro (below) set an Olympic 10,000-meter record under hot, humid conditions (28C with 60% relative humidity). She ran fast enough that sixteen years later, her time (31:01.63) would still have put her in the top ten in the much more moderate temperatures of the 2012 London Olympics.

Legendary coach and Nike cofounder Bill Bowerman once asserted that “there’s no such thing as bad weather, just soft people.” When it comes to heat, that’s not quite true, because if you are unprepared for heat, you will almost certainly do poorly competing in it. But I am hard pressed to remember a runner who is as terrible in heat as such experiences make them think they are. Almost always, their problem is that they’ve never really trained for it.

Sweating 101

The key to heat training is giving your body enough time in hot conditions for it to learn how to sweat more effectively. When you do that, the body makes a number of physiological adjustments, mostly involving the heart, blood, and skin. These are shown in box 1 below. Once you begin the process of acclimating, all of these changes kick in with remarkable speed.

BOX 1: TIMELINE FOR HEAT ADAPTATIONS^{Sports Med 2007. 37(8):669-682 (2007)}

 

A 2007 review of 77 prior studies found that some of these adaptations can occur in as little as three days. And all can occur within two weeks, assuming you are training consistently in the heat^{Sports Med 2007. 37(8):669-682 (2007)}. The bottom line is that the human body is remarkably adaptable to heat. According to Lawrence Armstrong, a heat researcher at the University of Connecticut, the body’s ability to adapt to high temperatures is faster and more dramatic than its ability to adjust to any other environmental stress nature can throw at it, such as altitude or cold.

Evolutionary biologist Daniel Lieberman suggests that our heat tolerance probably results from the fact that our ancestors evolved to hunt and forage in the middle of the day on the African savannah —a time at which they were relatively safe from less heat-tolerant predators^{Comprehensive Physiology 2015. 5:99-117}. Since then, humans have dispersed across the globe - but wherever we live, we still largely retain these ancient ‘heat-tolerance’ genes. The goal of heat acclimatisation is not to avoid lions on the savannah but to reawaken these genes in order to get the edge on other humans in summer races.

 

Embracing the heat

Conceptually, heat training is simple. All you need to do is to expose yourself to increasing temperatures and wait for your body to make the adjustments needed to run/cycle etc more efficiently in them. The simplest way to begin is by embracing each year’s heat as it arrives. On the first 15° day of the spring, that’s easy. Almost everyone wants to be out enjoying the warmth. But I tell my runners that they need to do the same at 18C, 20C, 25C, and even 30C. Our mantra is “Embrace the heat.” However, we also advise caution – for example if the weather suddenly spikes from 20C to 35C. I also tell athletes to be sparing in their use of air conditioning, especially in their cars. Sitting in traffic with the windows down and the air conditioning off isn’t the same as running intervals on the track, but heat is heat, and if you can get comfortable doing that, it will help you in racing and training.

Given the speed with which the body adapts, it’s possible to push this process quite rapidly. Brett Ely, a 2:38 marathoner with a history of running well in heat, once helped the U.S. Army develop a 10day protocol for acclimatising soldiers for deployment to Iraq, where temperatures can exceed 45C. But that doesn’t mean you have to be that aggressive. As Brett has described, it’s better to start gradually, rather than overheating yourself. This is like altitude adaptation; if you were going to high altitude, you wouldn’t do a hard workout on the first day - you’d ease into it.

The key is to make sure you stress the body enough so that it experiences enough stimulus to adapt, without excessive stress loading. And this doesn’t mean that you have to spend all day sweltering in the summer heat. Heat adaptation has been studied for a long time, and as far back as 1963, scientists determined that there is no advantage in extending heat-training workouts beyond 100 minutes^{Fed Proc 1963. 22: 704-8}. What’s more, 100 minutes is the upper limit; shorter workouts might not be quite as effective, but they will still move your body in the right direction.

In addition, heat-training workouts do not need to be super-intense. All that’s really necessary is to increase your core temperature and increase your sweating rate - something that doesn’t require intense interval training or anything close to it. Indeed, studies have shown that for heat training to occur, all that is needed is to exercise at intensities greater than 50 percent of your VO2max, which equates to an easy jog for runners, or a gentle spin for cyclists^{Sports Med 1991. 12(5):302-12}. Nor do you need to heat train every day. If you are in a hurry to acclimate, daily training will produce the desired results most quickly. But scientists have also demonstrated that in the long run, heat training once every three days for a month is every bit as effective, as heat training daily for ten days^{Int J Biomet 1975. 19: 41}.

How you put this into practice is subjective and may depend on how well you are already trained. The key is that you want to make yourself uncomfortable enough for your body to adapt, but not so uncomfortable you risk heatstroke or heat exhaustion. Self-diagnosing these is a bit difficult, but major warning signs include nausea, headache, confusion, dizziness, or hot, dry, flushed skin. A sudden sense of feeling chilled, even though it’s hot, is a crucial stop-now warning signal. You should also check occasionally to ensure that you’ve not stopped sweating—the ultimate red flag for incipient heatstroke. (A good way to do this is to touch the back of your neck and see if it’s damp.) If in doubt, it’s wise to conclude that you’ve had enough for today.

Sitting in traffic with the windows down and the air conditioning off isn’t the same as running intervals on the track, but heat is heat, and if you can get comfortable doing that, it will help you in racing and training

Acclima-what?

Effective heat training often involves using both acclimatisation and acclimation, as needed. If the weather is giving you what you need—and your work schedule allows you to take advantage of it—nothing beats simply getting out in the heat and using what nature provides. But if you’re trying to heat train and the weather (or your work schedule) aren’t being cooperative, don’t worry. Nike coach Alberto Salazar has been known to outfit his runners in rubber suits to prep them for temperatures more extreme than they could find at home. You can get the same effect by wearing tights and a long-sleeved top on a day in which you’d rather be in shorts and a T-shirt. Or, you can move indoors and do your training in an overheated gym.

Other factors may also affect your progress. For example, older runners may need more time to acclimate than younger ones…and may not acclimate as thoroughly^{Sports Med 1991. 12(5):302-12}. Similarly, athletes with high VO2max may acclimate faster than those with low VO2max, while sleep deprivation may reduce anyone’s ability to adapt^{Sports Med 1991. 12(5):302-12}.

It also helps to keep hydrated and make sure that you consume enough electrolytes – drinks with added electrolyte minerals are useful in these circumstances. But the bottom line is fairly simple: you need to target the middle ground – ie not so hard or long that you damage yourself with a heat injury, but not so cool/easy that you make no progress whatsoever. But with just a little work, it might be a short step to discovering that, like my runner and her hot-weather 5K personal best, you are actually quite good in heat.
 

Mild benefits

Finally, it’s worth mentioning that some pre-race heat training may be of benefit, even when you race in conditions that are not warm, but simply quite mild. This is especially true if your event is a long one – for example a marathon run or longdistance triathlon/sportive. For example, researchers for the US Army have found that even at temperatures of 10-15 degrees C, elite marathoners are slowed down by about 1-2 minutes compared to running in colder conditions. For less-speedy runners, this slowdown is significantly worse, amounting to 4-8 minutes for males running at 3-hour marathon pace^{Med Sci Sports Exerc 2007. 39(3):487-93}!

CASE STUDY: Kim Conley

British-born Kim Conley is an American 5K and 10K runner who competed in the 2012 and 2016 Olympics. She now lives in Sacramento, California, where summer temperatures in excess of 30C are the norm. But as the mercury climbs, she refuses to shy away from the challenge. Instead, she says, “I’ve tried to acclimate to heat by doing my workouts in the middle of the day.”

In addition, she gradually adds layers of clothes as the heat settles in. “It starts with simply not taking my shirt off for a workout, then moves to wearing capris and a T-shirt, to capris and a long sleeve, and eventually full tights and a long sleeve. These sessions can be very hard, but I definitely think it pays off on race day because in adapting to a warmer climate, your body becomes more efficient at cooling itself.”



However, the heat in Sacramento is mostly a dry heat, so, when the 2013 US national championships were held in Des Moines, Iowa (where heat often comes in tandem with humidity) Conley went to the American heartland for the last two weeks of her training. “My training is almost always geared around preparing for a specific racing environment,” she says.

Just as endurance athletes taper off their workouts before important races, however, Conley also backed off her heat training in the day or two immediately prior to the race. She trusted that the needed adaptations had already taken place and that what she now needed was to make sure she didn’t enter the race overstressed. It worked. At a temperature of about 27C and relative humidity of 60-70 percent, she ran 15:37.80, which although not her best ever time, was fast enough to secure a berth on the 2013 IAAF World Championships.