What effects does heat have on performance?
Last month, I wrote a piece on the altitude and the effects that altitude exposure has on training and racing. Much like altitude exposure acutely decreases endurance performance, so too does heat exposure. What is considered “hot” or “humid” is somewhat relative depending on what we as athletes are adapted or acclimated to. For example, someone living in Florida might think 60 degrees F and 50% humidity is a “cool” day, but someone living in Alaska coming out of the depths of their winter might think 60 degrees F and 50% humidity is scorching! Nonetheless, performance declines as ambient heat and humidity increases. An interesting study from 2007 noted that performance in the marathon began to suffer at ambient temperatures equivalent to ~40 degrees F, getting progressively slower as the temperature increases (1). Why does this happen?
Well exercising in warmer conditions, whether it is due to temperature or humidity, places additional strain on the body in the form of heat. The body has to constantly cool itself during exercise in order to maintain a stable core temperature, and exercise generates A LOT of heat. We cool ourselves during exercise through three primary means (3):
Convection – the transfer of heat from the body’s skin to moving air or water over the skin
Think of a breeze of wind cooling you down, the cooler ambient air hitting the skin allows for the transfer of heat from the body to the air
Conduction – the transfer of heat from the body’s skin to another cooler object that the body is coming into contact with
Think of dumping water on your clothing to cool you down; the cooler water creates cooler clothing, which allows for the transfer of heat from the body to the cooler clothing
Radiation – the transfer of heat from one object to another without any physical contact
Heat generated in the body is lost to the surrounding air
Evaporation – the release of heat with the evaporation of sweat from the body’s skin
This is the most obvious form of cooling to most athletes; evaporation works by allowing heat to escape the body as sweat evaporates into the surrounding air
These three methods of cooling work together to keep our core temperature stable during exercise. In cooler conditions, convection, conduction, and radiation provide the body with the bulk of heat loss for the maintenance of a stable core temperature (hence why you sweat less on a very cold morning run as opposed to a very hot evening run). However, as the temperature rises, our body must work harder and hard to keep cool, including diverting more and more blood flow to the skin for evaporative purposes as convection, conduction, and radiation become less effective. Additionally, as the humidity rises, evaporation becomes even more difficult as the surrounding air becomes more saturated with water molecules (this is why your clothing is usually soaked and dripping after exercising in hot and humid environments compared to hot and dry environments). While this happens, as temperature and humidity rise, more blood flow is diverted to the skin and less to the working muscles, causing a decrease in performance. This is somewhat of a simplification of how heat affects performance, but you get the point. This also explains why heart rate is higher at any given workload in the heat, because the body is working harder to cool itself in addition to maintaining the exercise intensity your set at holding. Any way you slice it, acute exposure to warmer and more humid environments than you are used to will decrease performance, with performance decreasing linearly as heat and humidity levels rise (2).
How long does it take to acclimate to the heat?
Fortunately, compared to altitude acclimation, heat acclimation seems to take place a bit faster. Studies have shown acclimation to heat in athletes in as little as eight days (2). Heat exposure leads to a host of physiological responses, but the primary driving force behind heat acclimation is an increase in blood plasma volume (2). How does this help an athlete handle the heat better? Essentially, it allows the athlete a greater reserve of blood volume to shunt to the body’s surface for evaporation to occur. It also helps athletes maintain a greater cardiac output during exercise at all intensities/outputs in the heat, thereby reducing cardiac strain and the subsequent elevated heart rate that one would see if not acclimated. This makes it easier to cool yourself while exercising, and this has a positive effect on performance. Not only will acclimating to the heat improve one’s performance in the heat, but it will elicit an improvement in cooler environments too. For this reason, elite athletes often use heat acclimation protocols leading into peak races in the same way they might use altitude exposure. The body adapts to the heat and then performs better at cooler conditions they might be racing in. For example, improvements in VO2 max of 13-23% have been reported in individuals in cool conditions after eight days of exercising in the heat (4). Additionally, increases in running performance indicated by a 32% increase in time to exhaustion in an time trial at current best 5k pace/speed (an ~2% improvement in performance in an endurance event) have been noted following three weeks (~12 sessions) of a post-exercise sauna protocol lasting ~30 minutes (5).
How to approach training and racing in the heat?
When it comes to training in the heat, follow the guidelines below to maximize your adaptations to the heat while preserving overall performance and fitness:
Exercise indoors or in cooler environments for key training sessions or training sessions with lots of intensity. By doing so, you will hopefully be able to hit faster paces than you would if doing these sessions in the heat, thereby preserving the integrity of the training session and maximizing the adaptive response that ensues.
Perform easy training sessions and aerobic training sessions in the heat to allow adaptation to occur. However, keep in mind you will need to adjust your pace/power to keep your heart rate in the correct training zone, otherwise you risk training too hard and turning an easy session or an aerobic session into a more intense workout than intended.
When it comes to racing in the heat, follow the guidelines below to maximize your performance come race day:
Allow at least 8-10 days of prior heat exposure to allow heat acclimation to take place. Ideally, you would be living in the warmer conditions that you would be racing in. However, even a few post-exercise sauna sessions or warm bath sessions lasting 20-30 minutes can do the trick. You can also adapt through doing easy and aerobic training sessions in the heat as outlined above to allow acclimation to occur.
To maximize your performance on race day, you’ll want to keep your body as cool as possible. This means you will want to make sure you have a proper hydration strategy in place, you are wearing the right clothing to maximize evaporation (i.e., wearing lightweight, sweat-wicking clothing as opposed to cotton), and taking advantage of other modes of keeping yourself as cool as possible (e.g., dumping water on your skin and keeping ice in your race kit). To learn more about what science says are the best ways of keeping cool while exercising, see this post here.
Obviously, I could go on and on about this topic as what I’ve discussed here is just scratching the surface of what could be discussed regarding heat and its impact on training and racing. However, the discussion herein should provide you plenty of background information so that you are equipped in best-handling the heat, including 1) knowing what to expect with acute heat exposure, 2) understanding how the body acclimates to heat exposure, and 3) handling training and racing in the heat.
1. Ely MR, Cheuvront SN, Roberts WO, Montain SJ. Impact of weather on marathon-running performance. Medicine & Science in Sports & Exercise. 2007 Mar 1;39(3):487-93.
2. Saunders PU, Garvican-Lewis LA, Chapman RF, Périard JD. Special environments: Altitude and heat. International journal of sport nutrition and exercise metabolism. 2019 Mar 1;29(2):210-9.
3. Vella CA, Kravitz L. Staying cool when your body is hot. Available at: https://www.unm.edu/~lkravitz/Article%20folder/thermoregulation.html
4. Shvartz E, Shapiro Y, Magazanik A, Meroz A, Birnfeld H, Mechtinger A, Shibolet S. Heat acclimation, physical fitness, and responses to exercise in temperate and hot environments. Journal of Applied Physiology. 1977 Oct 1;43(4):678-83.
5. Scoon, G.S., Hopkins, W.G., Mayhew, S., & Cotter, J.D. (2007). Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport, 10(4), 259–262.
Happy training and racing!
-Ryan Eckert, MS, CSCS