This summer Waco set a record high temperature of 114 degrees Fahrenheit (45.6 C). As an Indiana native who’s done the majority of his training in the cool breezes of the Great Lakes region, “heat is the poor man’s altitude” was my mantra this summer. I spent my summer repeating it to myself over and over as I cranked out mileage.
Heat is a poor man’s altitude.
Some of us have heard this phrase before, but is it true? Can the very condition that people work hard to avoid all summer long be equal to the condition that pro runners base entire training camps off of? Can a summer here in Waco really train us as well as the cool mornings of Boulder, Flagstaff, or Mammoth Lakes?
The answer to that question is multifaceted. Because of this fact, this article will cover a breadth of topics, so feel free to skip to the headers that interest you.
Why Does Heat Slow Us Down?
Our bodies are unbelievably fine-tuned to function within certain parameters. One of these parameters is temperature. If we get too hot, then the body’s systems no longer work as they were designed to. The enzymes that drive processes in the cells begin to denature (break down). When this happens, the enzymes can no longer make essential chemical reactions occur, resulting in a quick deterioration in the ability for major organs (think heart, brain, etc.) to work.
Since overheating can be catastrophic, the body uses thermoregulation. Thermoregulation is how our body senses and maintains a constant internal temperature. If we start to get too hot, the body removes heat through perspiration, a process commonly referred to as sweating. Perspiration works because as your body releases sweat droplets onto the skin, the sweat carries heat from beneath the skin to the surface of the skin. Now that this heat is no longer within the body, the body becomes cooler. Once on the skin, the sweat then evaporates into the air.
The problem with sweating though is that as you sweat, you use up the fluid from within the body. The dehydration from this fluid loss can cause significant drops in running performance. Research has shown that if the body sweats out even 2% of body weight, performance decreases by anywhere from 4 to 6 percent. This may seem minor, but I challenge you to think of it this way. If a 150-pound (68kg) runner previously on pace to run a 3-hour marathon loses just three pounds, they are now on pace to run somewhere between 3:07 to 3:11. This is enough to cost them a Boston Qualifier. Not even Nike’s legendary 4% shoes can make up this difference every time. To give this even more perspective, the difference between 1st and 20th at the last men’s Olympic marathon trials was a comparatively measly 2%. It’s a big deal. Furthermore, 3 pounds of bodyweight lost isn’t much. It can easily be stated that athletes could lose much more during the course of a normal run.
Compounding these effects is the fact that perception of effort being exerted is affected by humidity and temperature. When temperature and/or humidity rise, so does heart rate. Once all of these factors begin to compile, performance suffers.
As sweat, and thus water, is lost, blood volume decreases. With less blood in the system, less blood returns to the heart. Since less blood returns to the heart, there is less blood to carry oxygen to the working muscles. This causes the muscles to have a decreased ability to produce energy aerobically, which shifts the balance between aerobic and anaerobic energy production. Now, what is usually a comfortable, sustainable aerobic pace becomes a less sustainable, more anaerobic effort. The runner then slows to down to compensate.
Further complicating this matter, as the body gets hotter, it must redirect some blood flow from the muscle to the skin to dissipate heat through perspiration.
Though they have nothing to do with the cooling process itself, red blood cells (the cells which carry oxygen) are brought to the skin with the rest of the blood. This means that oxygen, rather than going to the working muscles, is going to the skin. In result, the runner has less oxygen, and thus, energy to use for running. Sounds a little like altitude, doesn’t it?
Humidity Vs. Dry Heat
Our body reacts quite differently depending on the exact reason that it is heating up. A humid day in Florida will cause a different bodily reaction than a hot day in the dry deserts of Arizona.
Humidity is a little trickier to acclimatize to. This goes back to perspiration and evaporation. Since the air has more liquid already in it when humid, less sweat evaporates. So then, rather than the sweat and heat going into the air, it remains on the skin. When this happens, there are adaptations that can occur to help the body deal with this, but its largely unavoidable.
One study design showed this concept best. Researchers had athletes exercise in a hot, dry environment. During exercise, the athletes had one arm covered in plastic, while the other was left uncovered. This effectively created a humid environment on one side and a dry environment on the other side. Researchers were then able to control for many factors by putting the same person through humidity and dry heat training simultaneously. After the heat training protocol was complete, they had the participants return to the lab. This time, they left both arms uncovered. The results showed that the arm that experienced a humid environment was not as adapted to the heat as the one that had been in the dry environment the entire time.
Acclimatization Protocols
Since our body reacts to humidity differently than dry heat, one of the most crucial parts of acclimatization protocols is being in, or trying to imitate, the environment that you are acclimatizing towards. Otherwise, you may get adaptations, but either not enough of those adaptations or the wrong adaptations all together.
Another core tenant in acclimatization protocols is that they are based entirely off of raising the body’s core temperature (the temperature inside the body). This rise in internal temperature is what triggers the issues and adaptation pathways when we exercise in heat, thus that’s what we need to induce when we are trying to acclimatize.
The most common, and ideal, way to approach a heat acclimatization protocol is to try to get into the environment you are training for. This typically happens naturally as summer comes along and temperatures rise or you move somewhere new that’s warmer than what you are used to. Let’s pretend you are an elite athlete though. You are preparing for a race in a hot location; say Tokyo Olympics 2020. Many are predicting the daytime temperature will hover around 100 degrees F (38 C). If this ends up being the case, it would be ideal for you to do your heat acclimatization protocol onsite in Tokyo. Think of it as a sort of specificity of training. Train in the environment and you’ll be ready for the environment.
Heat acclimatization takes about 7 to 14 days. As with most adaptive processes, people react differently. Some people may adapt quicker than others and some people will adapt to a greater degree. During this week or two of heat acclimatization, the athlete should spend about 90 minutes exposed to the heat at a time. This will provide enough time for the core temperature to rise and sustain a temperature that will trigger adaptation. As you are getting used to the heat during the protocol, it’s best to take it easy to prevent yourself from overtraining. With this said though, if you plan to race in a hot environment, it is wise to train at a comparable intensity at least once before race day, if possible. One other way to prevent over doing it is to gradually build up the temperature over the course of days or weeks. Unfortunately, most of us have no control over the environment we train in. We just walk out the front door and see what the weather throws at us.
There have been talks of designing a quicker protocol for athletes that would take 3 to 5 days. With this said though, there has yet to be evidence for such a protocol.
Anyone who is fit or participates in any exercise already has some heat adaptation. For this reason, those who are extremely fit, particularly aerobically, will be able to acclimatize quicker than individuals who are less fit. They will also tend to naturally handle hotter environments better, even when at rest.
Adaptations That Occur
The adaptations that occur center around driving perspiration to keep the body cool. The results of these adaptations are what primarily drives the improvements in performance.
One adaptation is a decrease in core body temperature during exercise. If you’ve been paying attention, this should be expected. Rising core body temperature is what the body works so hard to combat. It’s the stimuli which drives adaptation in the first place. It’s what the body sees as its enemy. A decrease in core body temperature aids in performance because the body no longer perceives the environmental heat as quite as large of a threat. As an indirect result, you also begin to consciously perceive the heat as less of a threat. You are now more relaxed and focused elsewhere.
One key adaptation which helps both performance and to decrease core body temperature is an increase in plasma volume. Plasma, in layman’s terms, is the fluid which makes up about 55% of the blood. With an increase in plasma volume, the skin blood flow improves. More importantly for performance though, there is now more blood to go around. If you remember earlier in this discussion, you will remember blood going to the skin steals oxygen away from the working muscles. This is now less of a factor since there’s enough blood to sustain adequate flow to both skin and muscles. The runner can then sustain an aerobic state at a faster pace than before heat acclimatization.
Another key adaptation is that the body responds to exercise and heat by initiating sweating sooner. Rather than waiting until the core body temperature significantly rises, the body will sweat at a small increase in core body temperature. What this allows for is a more proactive approach to thermoregulation. The body is now ahead of the problem, rather than dealing with it as it happens. This reduces the overall stress.
Similarly, sweat rate in general increases. This allows for heat to be extracted and dissipated at a quicker rate.
Heart rate, which can get quite high during heat, also decreases with adaptation. A decrease in heart rate is critical since it is a direct indicator of relative intensity. When exercise is intense, the heart works harder and heart rate goes up. When the heart rate drops, we know that the exercise is no longer as vigorous of an effort as before adaptation occurred.
Ways to Artificially and Safely Induce Adaptation
Just as you can mimic altitude conditions for adaptations at normal altitudes, heat training has its own set of techniques. In fact, mimicking heat training is far easier and cheaper than trying to create low oxygen environments. While altitude training has its altitude tents, heat training also has its own way to modify a single room. For this, people seek to make a single room of their home much hotter and train in it. Usually this takes a very DIY approach. People will often put plastic over the room’s doors and windows, place space heaters in the room, and begin to crank up the heat. As you can imagine, it is possible to get the room quite hot. Once hot, they will train in it and induce adaptations comparable to training in the heat outdoors.
For a less drastic approach, overdressing has begun to be used. In this technique, runners will dress for their runs in outfits that are much warmer than they need to stay comfortable. For example, even though it is a nice 65-degree F (18 C) air temperature, they will wear long running pants and a windbreaker over their normal outfit.
This technique is one that East African runners have been doing for quite some time. Kenyan runners have been known to wear long sleeves year-round, only changing into shorts and a t-shirt for workouts. I have personally tried this technique over the last two springs. In my opinion, both times have been very successful. From the standpoint of perception alone, I have felt much more comfortable in the heat once summer rolls around. I feel comfortable when I normally wouldn’t. I also seem to adapt to extreme heat much quicker than I used to. For example, temperatures that normally would have made me push runs back until long after dark I am able to do during the day.
This technique has stood up to the scrutiny of research. Studies have shown that overdressing sufficiently raises core temperature, heart rate, sweat rate, and skin temperature. As previously mentioned, this sets us up for adaptation. Notably, researchers have shown that overdressing alone has been sufficient to improve race performances in runners, both well trained and not.
Another technique that has been looked at is jumping in a hot tub or wet sauna post-run. This has been shown to have some benefits, but also some flaws. The key flaw is that by nature, both are very humid, or in the case of the hot tub, just plain wet. As discussed in the humidity section, the adaptations made in humid environments are capped. The body only adapts to a certain point. With the humidity, the core temperature can rise, but the processes surrounding perspiration are limited due to the lack of evaporation. For this reason, hot tubs and saunas are good, but not great. Also, from personal experience, this technique is not comfortable. While training for a marathon during summertime in the deep south, a friend and I used to occasionally jump in a hot tub post-run. It always felt awful. You couldn’t sweat and you felt nauseous.
The alternative to this would be a dry sauna. Saunas, at least in the States, aren’t very popular or common. Most of us don’t have access to one. If you do, and are so inclined, dry saunas can be beneficial to making heat adaptations. I would suggest you give it a try. Just be sure to report back to us how it goes!
Misunderstandings
Two common misunderstandings that must be dismissed are that dehydration can lead to heat adaptations and that once heat adapted you need less hydration. Both are wrong.
Our body is amazing at adapting to so many different stimuli that are thrown at it. Nearly everything we can adapt to, at least, to some extent. Because of this, sometimes people will get greedy with what they use to trigger adaptation. One such example is using dehydration as a training tool. As of now, there is no evidence that forcing yourself into any level of dehydration will ever improve performance. There is evidence though that frequent dehydration can be a stressor and eventually hinder the body’s performance, even after rehydration.
Back in high school, a friend told me that dehydrating yourself until a big race would help improve performance and make you less susceptible to slowing in the heat. Being young, uneducated, and generally naïve, I took him for his word. I would purposefully dehydrate myself for long periods of time (think weeks or months) until immediately before a big race. Then, I would rehydrate myself to normal levels. Often, I indeed would see a time improvement. Once I got older and wiser though, I realized the improvement was because I spent all season slowing myself down, then suddenly let myself run normally. It was as if I had raced all season in clunky cowboy boots, then come Conference switched into my normal spikes. Wearing the cowboy boots all season wasn’t what made me faster, it was finally taking them off that was. This was idiotic, dangerous, and didn’t make me faster, so don’t believe any suggestion you hear to follow a similar protocol. You should not, no matter what some coach or friend tells you, use dehydration as a training tool.
Another common misconception is that once heat adapted you need less water. This is simply not the case. Though you may feel better when running, you still need to consume lots of water when training in the heat. Rather than needing less water, you actually need slightly more. Just think of the adaptation that occur after a heat protocol. Your plasma volume increases, as does your sweat rate. What this means is that you now have to sustain a greater blood volume and you are losing more water during your run than before. Since you are sweating more, there is more water to replace. Even after heat adaptation, loss of body weight due to water loss is detrimental. Thus, you still need to try to maintain fluid levels as you run and throughout each day.
How to Modify Your Training in the Heat
Training in the heat is quite simple, yet runners struggle with it. The key is to just slow down and listen to their body. The problem with this is that runners, particularly in the USA, are culturally wired to hammer out as much mileage as they can as fast as they can. Without digressing from the current topic, this mindset can cause issues. They will often run themselves into a hole unnecessarily. When it heats up outside, we are already working harder, so slow down and don’t force the muscles into too high of an intensity.
I encourage you to check out this calculator to play around with how much running pace should differ by pace (https://hansonscoachingservices.com/hmmcalculator/race_equivalency_calculator.php). For example, I input a 5-minute mile at 55 degrees F (13 C) vs a mile at 95 degrees F (35 C). The result was that I should be expected to run between 5:24 and 5:45 for an equal effort mile at the increased temperature.
Coaches, I would advise you to use this knowledge when picking paces for workouts. First off, it would be wise to have athletes aim for a wider range. This allows them to listen to their individual body. Everyone reacts to heat differently. Second, I would suggest that you slow down the target paces. They will be doing the same effort, just moving a tad slower. As they become more adapted, then you may start speeding pace up gradually.
When it Cools Off Again
Unfortunately, heat acclimatization has fairly short-lasting effects. Though they will remain for the entirety of the time spent in heat, they fade quickly when you go back to cooler temperatures. In fact, nearly all effects may fade just as quickly as they came – two weeks.
Fortunately, heat adaptations can be maintained fairly easily. As long as the athlete returns to heat every other day or once every three days, the adaptations will stay intact. This is good for two reasons. The first, simply, is that it gives coaches and athletes a way to hold on to their gains. Secondly, it means if a tad bit of cold weather blows in it will not ruin you for when the heat returns.
Another piece of good news is that it will take much less time to adapt to the heat if you’ve done it well before. This means that if you take your time and adapt fully to heat your first summer in a hot locale, the second summer’s adaptation process will be quicker and easier than before.
So, is Heat a Poor Man’s Altitude?
From a performance enhancement perspective, this is the big question. Does training in the heat improve performance by a significant margin?
As we have already hinted at, a heat acclimatization protocol makes a huge difference when it comes to functioning at a high level in the heat. The adaptations that occur make a significant difference. Research has shown very consistently that athletes who have undergone heat acclimatization perform better than those who haven’t.
The data shows that VO2Max improves by 3-5% in cool conditions and 4-8% in the heat when compared to before heat adaptation. Though VO2Max isn’t everything in running, it is a rough predictor of performance. Time trial performance improvements have been recorded at levels as high as 6% in the cool weather and 8% in the heat when compared to prior to heat acclimatization. Additionally, there is some evidence that athletes who have undergone heat acclimatization can tolerate higher core body temperatures. What this means is that these athletes can sustain higher running velocities when their core temperature is at high levels. Overall, performance is improved.
Research has shown us that the adaptations that athletes get from training in the heat are transferrable to cool weather environments. This means that if an athlete trains in the heat, then goes to race somewhere colder, they will be better off than if they had been in cool weather the entire time. The benefits of adaptations in plasma volume, thermoregulatory function, etc., are not limited to just hot environments, but anywhere. Much like altitude, those who train in hot environments have advantages over those who train elsewhere.
Also like altitude though, training in the heat does not make up for poor training. The benefits of heat are not robust enough to replace a well-rounded training regimen with long runs, workouts, and appropriate recovery. Don’t expect to just become a pro overnight after completing a heat acclimatization protocol.
Further Studying
If you are still interested in heat training and want to dive deeper, here are some resources to give you a start. These won’t be everything, but they will get you thinking and lead to many more resources.
Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports
Episode 30 of Science of Ultra with Shawn Bearden, PhD. (Podcast)
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