by Tim Curry, MS ACSM-RCEP

Like many endurance athletes, I look forward to and dread the hot season. Peak mountain biking season and rising temperatures collide. Throughout July and the subsequent months we encounter temperatures in the 90’s to 100’s and consistently sunny days. High temps and a lack of shade make outdoor endurance activities like mountain biking anything from unpleasant to downright dangerous. This is not an unusual situation for endurance athletes who participate in outdoor sports, so how should we prepare for and handle training and competing in the heat? Let's dive in and explore!


What Happens in the Heat

At the very basic level heat training is all about managing heat production and heat dissipation. When heat production is greater than heat dissipation we run into problems managing our bodies’ heat. To explore both sides of this equation we need to start with four basic terms related to heat:

  • Conduction - The exchange in heat between two bodies that are physically in contact. 
    • Imagine how a hot bike seat transfers heat to your backside when you sit on it (and how quickly that makes you stand back up!).
  • Convection - The exchange of heat within a fluid, such as air or water. 

    • When you walk outside during a really hot day the heat we feel in the shade is the transfer of heat through the air into our body. 

  • Radiation - The transfer of heat through electromagnetic waves, without involving particles. 
    • Consider the hot day outlined above for the term convection, and how much hotter it feels when you step out of the shade into the sun. The sun is emitting electromagnetic waves that also cause our body to heat up. 
  • Evaporation - The process of a liquid changing from a liquid to gas state. 
    • This change in form (liquid to gas) requires energy which, for sweat, is provided by our body in the form of heat. 


When we train outside (or even inside) we are going to experience all four of these effects along with one more, which is actually the most important, metabolic heat. While conduction, convection, and radiation can increase our body temperature they pale in comparison to the heat produced due to metabolic processes. In fact, the majority of the energy released from metabolism is released as heat! This means that during exercise the largest dial we have to turn to change our bodies heat is the amount of metabolic heat we produce. The harder you go, the more we create. 


Fortunately, our body has a great way to cool itself. We call it sweating but it’s more correctly known as evaporative heat transfer. Our bodies push water to the surface of the skin where it can absorb excess heat, subsequently changing its state into a gas. The excess heat is then dispersed into the air. Humans are really, really good at sweating, but there’s an environment where this process is hindered. In humid environments it’s very difficult for sweat to become a gas, so it heats up but stays on the skin. This is counterproductive since we rely on sweat evaporating to help us stay cool and can quickly become an issue as we struggle to regulate our temperature.


The Problem with Heat

Everyone reading this has likely exercised in the heat and felt its effects on our body, even if those effects didn’t become a threat to our health and well-being. When our bodies have excess heat one of the first things we do is redirect blood flow to the skin and away from muscles and some organs. This helps with temperature regulation through convection and the sweating process. While these are beneficial to thermo-regulation there are some negative performance effects as well. 


When blood shifts away from muscles the amount of blood our heart pumps every minute (called cardiac output) must increase to continue supplying our muscles with the oxygen and fuel they need. Our cardiac output is determined by our heart rate multiplied by our stroke volume (the amount of blood your heart ejects in one contraction). For most training intensities you cannot increase your stroke volume so the only remaining option is to increase heart rate.This is one reason why during a hot day your heart rate can climb and stay higher than normal. 


Unfortunately, this effect also means your maximal performance is going to take a hit. When the body is unable to meet the cells’ oxygen demands (due to a lower total blood flow) you’ll hit your performance limit sooner. As we near our performance limit our cells must make more efficient use of the oxygen they have for aerobic metabolism. This means they begin using your limited carbohydrate stores sooner and consume them quicker. Once these stores are gone your body will quickly enter into power-saving mode by way of the dreaded “bonk”. There is nothing worse than thinking that you are going to have a great race or ride only to hit the wall by surprise!


Preventing Heat Injury

I bet you’ve heard of heat stroke, heat exhaustion, and heat cramps, but did you know these are different levels of hyperthermia (body heat is too high). Heat cramping can be unpleasant, but heat exhaustion and its more sinister cousin heat stroke pose a real risk for the health, safety, and well-being of individuals. We should always take precautions to prevent these issues. This can be as simple as a little bit of planning.


The first, and easiest approach to heat management is focusing on hydration. Aim to increase water intake during strenuous activities and ensure adequately hydration prior to your events (this does NOT mean over-hydrated!). A simple and effective way to judge your hydration level is to check your urine color on your next bathroom visit. Aim for urine that is pale yellow in color prior to starting your activity. You should consume enough fluids during your activity (and after) to maintain this pale yellow color. 


Another effective approach to heat management is a technique known as heat acclimation. The goal of heat acclimation is to increase an athlete’s sweat rate, start the body sweating sooner, increase the blood plasma volume (which is mostly water) and improve pacing in hot environments. This technique is effective, but can be challenging to do effectively and requires a bit of invested time. Unfortunately, simply being in a hot environment isn’t gonna cut it. 


Possibly the most effective acclimation approach requires ~8 to 10 days to complete, with ~1 hour of work per day. During these ~1 hour periods you’ll need to go out and train in the heat at a pace that elicits a rating of perceived exertion (RPE) of 3 or 4 out of 10 on the modified Borg RPE scale within 5 minutes of starting; a moderate to somewhat hard pace. If you’re referencing a recent personal VO2max test, aim for a pace equivalent to 50-60% of your VO2max. This pace is easiest to maintain consistently if you are running or have a cycling power meter. For use out on the trail, aim for a fairly flat ride where you can easily maintain a consistent speed. Later in your activity, your RPE should begin to climb. Stop the activity once your RPE reaches an 8-9 out of 10. This change in RPE results from the heat stress you are accumulating while training in a hot environment. The tricky part of this technique is coordinating the end of the outdoor activity to correspond with the appropriate RPE. You’ll need some logistics planning to ensure you can get out of the heat quickly, and start recovery ASAP. Make sure to drink plenty of water, both during and after these sessions, and keep an eye out for the signs of heat injury (heat-cramps, -exhaustion, -stroke). 

If you find you don’t have the time or ability to complete an active acclimation plan, but you have an event in a hot environment, there are other solutions to try. For short to moderate but high intensity events you can try pre-cooling the body (especially the skin) prior to the event to help ward off some of the effects of the heat. Focus on cooling the “core” of the body. This can be done with anything from ice vests to nylon stockings filled with ice and dunked in water. It’s important not to cool the active limbs, such as the legs for cyclists. 


Athlete or not, we all know the feeling of grabbing a cooling slush beverage from the convenience store when the temperatures start to rise. There’s been recent investigations into applying a similar concept to established cooling protocols with the addition of “ice slurry” drinks (a mixture of small chunks of ice and water or sports drink). The idea is to focus on cooling the “core” through internal methods in conjunction with external methods such as with ice vests or similar. While some of the data looks promising it is also important to include a word of caution here. It’s not just a brain freeze you need to watch out for. Ice slurries appear to need to be administered at a very specific amount and rate to be effective. It’s possible to consume too much too quickly and negatively impact your thermoregulation. The hypothesis is that excessive cooling efforts will disproportionately delay the onset of sweating. This can cause a chain reaction where the body rapidly overheats as the limbs get hot before the body’s core eventually catches up. If you do decide to experiment with this technique start off slow in a safe environment and find the amount that works best for you. 


Taking It To The Team

How can we put these ideas into practice with our teams to improve heat safety and training? Lets start with the simple items. Begin by educating athletes about hydration guidelines and consistently encourage hydration efforts during practice. This might mean you will need to send athletes who show up unprepared home, but a disappointed athlete is better than one with a heat injury. Next, we can plan practice schedules around the environment we are in. Use tools like the wet bulb globe temperature, and heat index along with NICA guidelines to assess heat risk for your area. When training in hotter environments it’s best to help mitigate risk by reducing intensity, increasing breaks, and attempting to pick training rides with adequate cover. The most important aspect here is training intensity! That may mean picking a ride that is far easier than normal. 


If you know racing in high heat is likely to occur then consider how you might help your athletes with heat acclimation training. This may or may not be appropriate for your team and athletes. Therefore, it is important to ask if you can effectively execute the plan without increasing risks to your athletes. Safety first! Focus on educating your athletes and coaches on the signs and symptoms of heat illness. A good place to start is this fantastic CDC infographic which includes what to look for as well as what to do. When it comes to heat it’s always better to err on the side of cautious than risk heat injury. 


With a little planning you can help prepare yourself, your athletes, and your team for success training and racing in the heat! To learn more about training intensity and endurance training, visit the courses found below.