How My Heat Workouts Paid Off This Summer (and What I’ll Do Differently Next Time)
Running in the heat is never fun, but this year I was determined to make the most of it. Starting in the spring, I used a Core 2 sensor to track my heat exposure; later, I added a Garmin Forerunner 570 watch , which gives me a heat acclimation score. I was recently struggling through a 90-degree trail run on a humid August day, slowed down by the heat and the hills, and I couldn’t help but think that all my heat training hadn’t made it any easier. Then it hit me: I was running in 90-degree heat . I’d never done that before.
Training in the heat didn’t make running fun, but it did make those grueling runs possible. This time last year, I was running five miles, and that was on an air-conditioned treadmill . This summer, I’ve been running 10 miles almost every weekend. Outside. Often during the hottest part of the day, because that’s how my schedule works.
I’m impressed with how much better I’ve gotten at tolerating the heat this year, and it’s been fun to watch that progress through one or both of the gadgets I’ve been using to track it. However, now that I’m experiencing the summer heat almost every day, the gadgets have become less useful, and I’m realizing both the upsides and downsides of the process I’ve been through.
Next year I will definitely do some kind of training, but I’m not sure it will look the same. So let me tell you how it went for me and what lessons I learned.
I started my warm-weather training in the spring.
As I wrote in my Core 2 review , I started training in the heat when the weather was still pretty cool. The Core 2 is a gadget that attaches to a chest heart rate monitor (and yes, you do need a chest heart rate monitor for this device to work properly).
In the spring, an early morning run in cool weather didn’t earn me any heat acclimation points. But warmer runs helped a little, as did longer indoor runs. I could also improve my Core score by going to the sauna after a treadmill run, so I tried to incorporate that into my program a few times a week.
One thing I noticed right away — and heat training experts will tell you this openly — is that heat training, including sauna use, is incredibly draining. Even if the workout is light (or even if you ‘re sitting in a hot room doing nothing ), it’s still stressful and your body needs to recover from it.
I felt it. A 40-minute easy run plus 20 minutes in the sauna sent my heat acclimation score through the roof, but I was completely exhausted. I could have spent the same amount of time and energy on a 60-minute run with some intense intervals, and it probably would have done more for my overall fitness. But instead, I chose to spend those resources preparing for the heat. Honestly, I’m still not sure if that was the right decision. It’s a trade-off, for sure.
I sweat more than before (but that’s good)
Believe it or not, you sweat more when your body is used to the heat. Beginners often think that sweat is a sign of poor fitness, but this is not true at all. Sweat does not reflect the intensity of your work; it is simply the body’s cooling process . The more intensely you work out, the hotter you get because your muscles generate heat as they work. So as you get ready to work harder in your workouts, you will need to sweat more to cool down.
Heat adaptation makes you sweat more . When you become more tolerant of heat, it doesn’t mean your body just gets hot and stays hot. Too much heat is dangerous, so your body needs to cool down in hot weather. A body temperature of 104°C (104° F) is considered heat stroke ; at high temperatures, your heart and brain can’t function properly, and overheating can be fatal.
The more you tolerate heat, the better your body copes with cooling itself. You sweat faster and more intensely . Your face may turn red as your body increases blood flow to the skin.
The first sign that my heat training was working came when I was sitting in a sauna one day. The other women, who had been there for about the same amount of time as me, were glowing slightly. I was sweating buckets, as if I were sitting under an invisible shower head. I felt awkward for a moment, and then I realized: my heat training was working .
My mental achievements were enormous.
Before I tried working out in the heat, my only thought about working out in the heat was, “Ugh, that’s gross.” But once I started intentionally exposing myself to the heat, my thinking completely changed.
In those first weeks, I found myself craving the heat. If I had the choice between a morning run in 21°C or an afternoon run in 26°C, I would have chosen the latter to better acclimatise to the heat.
Having the gadget track my heat acclimation was motivating. (Both the Garmin and the Core 2 do this, though in slightly different ways. I’ll get to that later.) On the Core 2, I could see that each workout significantly increased my status, while each day spent indoors with air conditioning decreased it a little. Looking at these graphs inspired me to run more often in warm weather.
I also just felt better about pushing myself. Whenever running felt grueling, there was a silver lining: I was getting used to it! In fact, the more the heat slowed me down, the more I realized it was good for me. It was much better than thinking, “Oh, it’s so hot and I’m slow.”
Now I pay more attention to overheating safety.
I started my journey into training in the heat with a review of Core 2, which isn’t a must-read for training in the heat—you can just run more in the heat—but it gave me a lot to think about. I interviewed Brian Maiorano, the trainer relations coordinator at Core, and learned a lot from him.
One of those “eureka moments” was why athletes are often advised to “pre-hydrate” rather than drink to thirst, which I tended to ignore. The body has a limit to how much water it can absorb by drinking water each hour—about 1% of your body weight—and you can easily lose 2% or more per hour if you sweat a lot. For short runs, this isn’t a big deal, but on longer runs in the heat, these losses can add up. Learning this has made me more mindful of my hydration.
I also became more aware of the possibility of overheating. I usually hide from the heat, so it’s not an issue. But now that I’m seeking heat, I need to be more careful about safety. I was a little concerned when I wrote the Core 2 review, especially when I explained the “heat strain index” zones. There are four zones, and generally, when training under heat, you want to be in zone 3. Zone 4 is too high.
As I was writing this, I remembered an interval run I did in the heat. I went back and checked my data from that run. I was pretty miserable throughout the workout, but toward the end, I couldn’t maintain my planned pace at all. It turns out I hit heat zone 4 right around the time my pace dropped into the toilet. Dumbass that I am, I kept running. (“I’m almost there, I can do this!” I kept telling myself.) I could see from the data that I stayed in zone 4 while I sat and drank some water afterward, and that I was still in zone 4 when I decided to swap my cool-down run for a cool-down walk. The moment I dropped into zone 3 was almost exactly, down to the second, the same moment I decided I felt good enough to jog again.
Takeaway #1: I’m impressed that the zone system actually seems accurate. Takeaway #2: What the hell was I doing continuing to train when I was hot and then insisting on running the entire day’s mileage when I knew I was too hot to run? I’m glad I decided to drink water, stay in the shade, and walk instead of run, but in hindsight, I should have avoided it. My car has air conditioning. Why didn’t I cut my run short and sit in the car to cool off?
Since then, I’ve become more safety-conscious. I pay more attention to how hot I am, and I now plan cool-down breaks if I’m going to be outside for any length of time on a hot day. For example, my first 10-mile runs of the summer would be split into five miles, then I’d sit in the air conditioning for a bit, have a snack and drink, and then head back out for the remaining five miles. It may not give me the same training stimulus as a straight 10-mile run, but on the hottest summer days, it’s a lot safer.
How Garmin and Core 2 helped (and didn’t help)
As I mentioned, I have two gadgets that track heat acclimation. One is the Garmin Forerunner 570 , which has a built-in heat acclimation metric.
Garmin uses weather data to estimate heat acclimation after outdoor runs. ( Here’s a Garmin page that explains how it works .) It doesn’t know how hot I personally am, and it doesn’t take indoor runs into account at all. It’s a pretty basic feature, but to be fair, its acclimation estimate generally matched up pretty well with what I saw on the Core 2.
Another gadget is the Core 2 monitor, a more specialized device that retails for $295 and attaches to a chest strap. The Core 2’s main selling point is the granularity of the data it can gather during your run (as I described above), as well as tracking the rise and fall of your heat tolerance over time.
The biggest downside to the Core 2 is that I only get this data if I remember to wear the chest strap every run and if the Core 2 is working properly.
There are a few caveats to this last point. First, the battery needs to be charged, in addition to making sure the heart rate monitor and my running watch are charged. Second, the Core 2 needs to be turned on. You can save battery life by putting the Core 2 into standby mode when not in use, but you’ll need to turn it on by shaking it before you start running.
I’ve definitely missed a lot of data by forgetting to turn it on. A few times, I’ve run about a mile and thought, “Wow, it’s hot! I wonder what heat zone I’m in?” Then I’ll scroll down to Core 2 and realize it’s not even turned on. Then I’ll turn it on and it ‘ll start collecting data. Every time that happens, the temperature on the unit seems ridiculously low for the next mile. So I can’t really trust that two-mile data.
One day, the device seemed to malfunction. It was a hot day, and I was surprised to find that the device thought I was nice and cool. I ran another mile, sweating profusely, and then collapsed on a bench in the shade. I looked at the watch again. It still thought my temperature hadn’t risen. I ended up ignoring it for the rest of my run, and then, when I got home, I tried to reset it to factory settings. Things worked better after that, but I was disappointed that the device hadn’t worked.
I also forgot the chest strap many times. There is no way to add a workout later (except for sauna), so the data simply wasn’t saved. Same with Garmin, but it’s much harder to forget a running watch than a chest strap, which I mostly left to dry in the shower.
To be honest, both gadgets were useful in the spring when I was just acclimatizing, but in the summer they are not so useful. Every time after a run, my Garmin shows that I am 100% acclimatized to the heat. Of course! It is August and +32 outside!
Likewise, I forgot to wear the Core 2 on my last few runs, but I haven’t looked at the app lately either. It’s hot today, it was hot yesterday, it’s going to be hot tomorrow. The heat acclimation data doesn’t help me much, even though I know I have.
What I learned for the future
My experiment is only two-thirds done. The first stage was about adapting to the spring heat. It went well, and both gadgets were useful (Core 2 is my favorite). The second stage was running in the heat, which went well, and the Garmin watch was more convenient because it is a little more reliable.
The third phase of the experiment will begin when I see what happens when the weather starts to cool down. In theory, training in the heat gives athletes an advantage in cool weather. (However, the science on this is mixed, and training in the heat may not make much difference to cool-weather competition.)
I don’t use the Core 2 often enough to keep its heat acclimation score up to date, but I like that I can wear it while running and get instant feedback on how much the heat is affecting me at that moment . This can be used to determine if you’re overheating, for example; another way to determine if you’re staying cool enough during a run or workout in cooler weather to avoid being overwhelmed by the heat.
I’m looking forward to what happens this fall. I’m thinking of running maybe a 5K or a half marathon in October and seeing how the cold weather affects my running speed.
And next year, who knows? I might use a heat-tracking gadget or I might not, but I think I’d like to apply my mental gains from spring training to help me better adapt to the heat and stay more positive during those slow, hot runs. I’ll also be more mindful of staying hydrated and staying safe in the heat. So far, the idea of training in the heat has definitely helped me; without it, I’d probably still be running short sprints on the treadmill instead of planning another half-marathon on the trail next weekend.