It’s a terrible feeling. You go out for a run on a beautiful, sunny day. You feel good and you’re ready to kill it. Before you know it, you’re a few miles in and you feel sluggish, your pace sucks, and you’re sweating like you just spent a couple days in the jungle. It’s runs like these that make you doubt your fitness and wonder if you are just a horrible runner after all.


Coming from a cycling background I never gave too much thought to how hot weather can dramatically impact your running. Running was just running and I simply chalked up situations like the one above as “bad days” and moved on. Unfortunately, that couldn’t be further from the truth.


Running (or racing) in heat and humidity is challenging. If you don’t understand some of the factors at play it can set you up for a world of frustration, or worse, land you in a medical tent. My hope is to save you the hassle and get you to a point where you are training and racing intelligently, despite the conditions.


Training in the heat? Be intelligent about it

Training in the heat? Be intelligent about it. [Photo courtesy of Pixabay]


Heating Power vs. Cooling Power

The best place to start is with a foundational question: How does hot weather affect running in the first place? In the simplest sense, it boils down to the idea of heat balance, which is a battle between heating power and cooling power.


The idea of heating power revolves around the inefficiencies at play in the human body. It starts at the chemical level. Movement happens when our bodies convert the chemical energy stored in adenosine triphosphate (ATP) into mechanical energy (muscle contractions). Unfortunately, up to 70 percent of the energy created from that conversion is released as heat. For every 1000 kCal we metabolize, only about 300 kCal create the actual forward motion of running. The rest of the energy is wasted as heat, which has to be removed otherwise it accumulates and we overheat.


That’s where cooling power comes into play. Our bodies have to maintain core temperatures within a narrow range (95 to 113 degrees Fahrenheit), otherwise they begin to shut down. Without a way to counteract the heat generated from running (or from the ambient air temperature), maintaining this range would be difficult. There are a lot of elements at play when it comes to counteracting all the heat:


  1. Air temperature is one of the more obvious factors at play. When it is cold, excess body heat is no big deal as it is easily dispersed into the colder air. Once the thermometer climbs above 55 degrees Fahrenheit, the game changes and the air temperature adds heat to the equation, causing blood to be diverted to the capillaries in our skin to aid in perspiration.


  1. Perspiration and evaporation. When sweat evaporates off our skin quickly, it has a wonderful cooling effect. And to a point, the more you sweat, the more it can evaporate, making this a powerful tool. When sweat doesn’t evaporate well, like when the humidity is 60 percent or higher, it doesn’t do much good and is more akin to a slimy, warm blanket you can’t shed.


  1. Wind speed (and direction). As nature’s fan, wind can be a godsend on a hot day as it helps cool your body. There is one caveat to wind, however: direction matters. Headwinds and crosswinds slow you down but they cool you off. Tailwinds, on the other hand, help your pace but make you feel like you woke up in a desert. Why does it work this way? It boils down to the difference between your pace and the wind speed. The greater the difference, the more cooling power at play. This is why overheating on a bike tends to be less of an issue.


  1. Shade and cloud cover. Anytime you can cool your body by catching a break from the relentless sun, do it.


  1. Other means. Things like wiping down with cold, wet sponges or drinking chilled fluids are also factors in the heat-balance equation.


Acclimate to the Heat

A good place to start to make the most of running in the heat is acclimating well and controlling the controllable.


Primarily a function of improving your sweat rate (so you can sweat more quickly), acclimation can help you improve your performance in the heat by up to 8 percent. It takes about 7-14 days of training in hot conditions to fully acclimate, so spend time training in the middle of the day to help the process along.


Perhaps you are preparing for a hot race but don’t have hot weather at home? No problem. Dial the heat up by wearing more layers or visit a sauna and spend a few minutes exercising (carefully) in there. Either way, prepare to sweat. A lot.


Control the Controllable

There is a lot outside of your control when running in the heat of the day. There are a few things on both sides of the heat-balance equation that you can control to train and race better. One of the most important is your pace.


The energy cost of running is independent of your running pace (but not incline or elevation gained). It takes about 1 kCal of energy per kilogram of body weight to run one flat kilometer, regardless of pace. So if you are 80 kilograms and run 10 kilometers, you’ll expend about 800 kCal. What gets a little confusing, though, is that pace does affect the rate at which heat is produced.


Let’s say it takes you an hour to run that 10k. That means you are generating about 560 kCal of excess heat per hour (70 percent of 800 kCal). If you run faster, that rate increases and vice versa. You can control how much excess heat you have to counteract simply by adjusting your pace, helping you avoid overheating.


One of the simplest ways to make this adjustment is to plan to slow your pace by 1.5-3% for every 10 degrees you are above 55 degrees Fahrenheit. This idea is courtesy of coach Jeff Galloway, and is simply an observation he made with himself and other runners. In my experience it has been spot on as well.


pace chart

[Chart courtesy of Shawn Gerber]


Another way to achieve the same end is by using heart rate. To aid in perspiration the body diverts some of the blood formerly used to fuel muscles to the capillaries in your skin for cooling. This lowers your available blood volume, so your heart rate must increase to maintain the same pace as you would in cooler temperatures. I highly recommend using heart rate in your training as it will help you to maintain the correct intensity regardless of the conditions.


If you think this adjusted pace seems too slow to get a good workout, you're wrong. Your body doesn’t know the difference in the pace. Go off of your heart rate and stick to your zones, otherwise you may be working at a dangerously high percentage of your maximum heart rate.


The last controllable in your arsenal is anything you can physically do to add to the cooling power. Drink cold fluids throughout the race, wear cooling fabrics, find shade and take advantage of ice and sponges at aid stations. Whatever you can add to the cooling side of the equation will buy yourself a little bit of extra speed out in the heat.


Win the Battle

Training and racing well in the heat is about being aware of your environment and making a solid game plan so that you can win the battle between heating power and cooling power. When others are on the side of the road breaking down, you will be the one pushing onward because you made smart, informed choices. Go get it! 


More hot weather training tips:

The Heat Can Beat You: Training Safe in the Summer

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