I know that we're knee-deep in race season at this point in the year, but I'm happy to report that I have just recently been diving into some longer distance runs. I have no particular races in mind. It's true, I've ran three races in April, but they were convenient in distance to my training, and looked like fun.
The unfortunate part of this for me is the timing: it's almost May. Significance? The North Carolina heat is starting to settle in and the fun factor of my runs is draining quickly. Historically, I perform extremely poorly in the heat. Up until 2011, I tapped out and typically took Summers off. In 2008 I tried my best to make it through the heat. I can recall numerous long runs in July, waking at 4AM, still 92 degrees outside, and hitting the pavement for 2-3 hours. Everything was going great until the thermometer began to swell.
I'm Dr. Allan Buccola, physical therapist and owner of Impetus PT in Greensboro, NC. I thought this would be a great time to write about how extreme heat affects the body in exercise. This blog post dives into why heat makes exercise so difficult and how to make the most of it. Like all posts, this one is intended to serve as educational only, not as medical or professional advice. If you have a specific question or medical concerns, please see a physical therapist or other medical professional to evaluate your case specifically.
Anatomical Basis for Heat Management
Scientists have been studying how the human body sheds heat for decades. As it turns out, humans are incredibly renowned at handling heat, when compared to other running species. The combination of hairlessness, the ability to sweat, and the fact that we need not rely on panting as a means of dissipating heat, gave our species a true upper hand on the plains of Africa. It's widely believed today that 'persistence hunting' may have been a primary means of acquiring food, essentially chasing down game for hours at a time until it collapsed from heat exhaustion.
More interestingly is the anatomical variation from humans that developed in hot, arid climates compared to those that developed in colder ones. The development of longer, slender limbs gave heat burdened humans a greater surface area to mass ratio, allowing for greater dissipation of heat, whereas cold climate ancestors maintained shorter, stocky limbs and trunks, better for maintaining body heat in extreme cold.
Physiological changes in heat
The human body is truly a finely tuned machine, and when it comes to metabolism, a finely calibrated chemistry lab. When temperatures change in the slightest, many of the chemical reactions that occur easily under normal circumstances begin to slow down.
Changes in Oxygen binding
As muscles continue to produce force in rising temperatures outside, the muscles themselves will begin to overheat, meaning that optimal blood temperature is lost . With this rise in temperature, the ability of oxygen molecules to bind to hemoglobin begins to decrease, meaning that you have to breath more, and work harder to keep your body oxygenated.
BLood volume changes
Perhaps the biggest factor of all is related to changes in blood distribution during activity. Under normal exercising circumstances, the greater your activity effort, the greater blood flow increases to the muscles. Trained athletes will develop greater blood volume over time, which creates an advantage. As the body begins to overheat, blood must be redirected away from muscles to the surface of the skin to help keep the body cool.
As blood shifts away from the muscles, the heart must beat faster and harder to maintain relatively equal supply to the working system. This means that as temperatures increase, so does your heart rate...at a lesser effort than in cooler temps. Essentially, your doing the same amount of work with a fraction of the available blood, and because blood is what keeps your muscles fueled on the run, your overall ability to perform work is diminished. This is the same effect that occurs with dehydration, and the with concept of 'cardiac drift' for those of you that are familiar with the term.
Body temperature parallels brain temperature
An area that is probably just recently beginning to be understood is how hyperthermic changes to the brain lead to decreases in motor performance and output from the central nervous system. Anyone who has run fever knows that high brain temperatures can cause brain damage at a certain point, so it should come as no surprise that many physiologists see this as a protective mechanism.
Many of the metabolic systems work in a similar way in that the brain will shut the body down, long before the body is able to put the brain in harm's way. This is the reason Olympic marathoners 'give it their all' during a race, but still have yet to run out of glycogen: survivalist tactics.
Gastric Emptying Decreases as Temperature goes up
Research has shown that as the body begins to heat up, the ability to drink fluids back into the body decreases. The stomach simply has a harder time emptying its contents. Also related, as exercise exceeds 70% of VO2 max, a common effort for most running less than 1 hour, a very similar effect occurs in the stomach. Paradoxically, as we have established that high temperatures increase work effort during activity, the combination of these two effects results in a fast road to dehydration unless activity modifications are made.
One can attempt to take in additional fluids, but ultimately will have tummy troubles. Under ideal circumstances, a rate of 600ml (~18floz) per hour of fluid is about the maximal amount the stomach can process during moderate exercise. This number decreases with additional effort and hotter temperatures. Now you know why camels have humps!
Prep yourself for exercising in the heat
Water, Water everywhere
Hopefully this comes as just a reminder, but as the temperature outside begins to increase, so should the amount of fluids you take in. Water needs are highly variable from person to person, but there are some well established guidelines in the research (I'll get to those in just a moment.) Keep in mind that in the hottest, most humid conditions, sweat rate can reach a maximum of 2.8L per hour. That's a lot!
If you're curious about whether you are keeping yourself adequately hydrated, the easiest way is to weigh yourself before and after the run. A loss of more than 2% of body weight is considered risky and more than 3%, dangerous. Two percent of body weight is approximately 3.5lbs for a 175-pound male or 2.8lbs for a 140-pound female. Consider rethinking your hydration strategy if you find your weight to fluctuate more than this after a sweaty workout.
There is a great paper from 1992 by Gisolfi and Duchman, that established some guidelines for fluid replacement during moderate to high intensity sports activities. Although not specific to high temperature activity, they recommend that activities less than an hour be matched with 16floz-1 liter of water, and activity of 1-3 hours in length be matched with 800-1600ml. Moderate activity bouts over an hour also warrant fluid replacement with some salt and sugar content, preferable a blend of glucose and fructose. Not all sports drinks are created equal, so do your research and know the difference.
One last note regarding hydration: on a hot run or workout, adequate hydration results in lower heart rate, lower perceived effort, and lower core temperature. Essentially, adequate hydration equals maximal performance.
Maximize skin exposure
The body's primary means of cooling in high temperatures is through sweat. The actual process of cooling occurs when the sweat evaporates from the skin. This means that sweaty skin needs to be exposed to air to cool down, so be sure to wear loose clothing and expose appropriate amounts of skin to the air to allow for maximal evaporation. This can be tricky because, direct skin exposure to sun, especially in the absence of sun-screen can make overheating worse, so finding a balance is key.
Give Yourself time to acclimatize
The body acclimatizes to heat in some very cool ways. One study shows that in as little as 10-14 days of moderate to vigorous activity in hot temperatures, the body begins to increase the ability to sweat, while it also becomes better at sweating with lower salt concentration. This adaptation slightly decreases the chances of dehydration with hot weather activity, but bear in mind that this accommodation is likely something that needs to happen annually for most people who have some version of winter where they live.
Follow Heart Rate for Effort, Not Pace
As heat stress, dehydration, and fluid volume shifts all contribute to greater required efforts in the heat, the greatest determinant of exercise capacity is heart rate. Regardless of the pace you typically run or ride, a single hot day can quickly slow your 7:45' mile average down to a 9:15' mile. Each person has their threshold, their personal ability to shed heat, and their own level of acclimatization.
If you don't wear a HR monitor, find ways to manually assess with palpation and a watch, or just pay attention to effort. Heart rate and perceived effort correlate well. I find that when I begin to approach 80% max HR, running begins to feel more like work than fun. That feeling in the heat might mean it's time to slow down.
Whether your sport is running, cycling, hiking, or anything else, if you're sweating hard, you've got to make accommodations to stay safe. I personally used to feel like I was losing ground in the Summer with slowing speeds and shorter distances, but don't be fooled. Pay attention to your effort and heart rate and you will maintain and continue to develop your fitness level. Ignore the concept of pace, and focus on learning how to hydrate well, because it can make you or break you on race day. If you would like more detailed and personalize recommendations for hot weather training, contact me to set up a consult. Until next time: don't stop moving!