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Running in the heat is never fun, but this year I was determined to make the best of it. Starting in the spring, I used a Core 2 sensor to track my heat exposure; I later added a Garmin Forerunner 570 watch, which gives me a heat acclimation score. Recently, I was struggling through a trail run in 90 degree heat on a humid August day, massively slowed by the heat and hills, and couldn’t help thinking that all my heat training hasn’t helped this to feel any easier. And then it dawned on me: I’m actually running in 90 degree heat. That’s not something I used to do.
Heat training hasn’t made hot weather running fun, but it has made those sweltering runs possible. This time last year, I did no more than five-mile runs, and I did them on a treadmill in the air conditioning. This summer, I’ve managed to get in a 10-miler most weekends. Outdoors. Often during the hottest part of the day, because that’s just how my schedule works out.
I’m impressed at how much better I am at surviving the heat this year, and it was fascinating to watch the process through one or both of the gadgets I used to track it. That said, now that I’m experiencing the summer heat nearly every day, the gadgets have gotten less useful, and I’m recognizing both pros and cons to the process I went through.
I’ll certainly do some kind of heat training next year, but I’m not sure if it will look quite the same. So let me walk you through how things went for me, and what lessons I learned along the way.
I started my heat training in the spring
As I wrote in my review of the Core 2 sensor, I began training for the heat while the weather was still somewhat cool. The Core 2 is a gadget that slides onto your heart rate chest strap (and yes, you do have to use a heart rate chest strap to use this device properly).
In the spring, an early morning cool weather run didn’t gain me any heat acclimation points. But warmer weather runs gained me a little, as did longer indoor runs. I could also boost my score in the Core app by doing a sauna session after a treadmill run, so I tried to work that into my routine a few times a week.
One thing I noticed right away—and that experts on heat training will openly tell you—is that heat training, including sauna sessions, takes a lot out of you. Even if the workout is an easy one (or even if you’re sitting in a hot room doing nothing) it’s still a stress that your body has to recover from.
I felt that. A 40-minute easy run plus 20 minutes in the sauna sent my heat acclimation score through the roof, but it left me exhausted. I could have spent the same amount of time and energy on a 60-minute run with some hard intervals, and arguably that would have done more to help my overall fitness. But instead, I chose to spend those resources on preparing for the heat. To be honest, I’m still not sure whether that was the right call. It’s a tradeoff, to be sure.
I sweat more than I used to (but that’s a good thing)
Believe it or not, you sweat more when your body is used to the heat. Beginners to exercise often think sweat is a sign of being out of shape, but it’s really the opposite. Sweat isn’t a measure of how hard you’re working; it’s just your body cooling itself down. The harder you work, the hotter you get, since your muscles are producing heat anytime they’re working. So as you become able to take on more of a challenge in your workouts, you’ll need to sweat more to cool down.
Heat adaptation makes you sweat even more. When you’re able to tolerate the heat better, that doesn’t mean your body just gets hotter and stays hotter. Excess heat is dangerous, so our body needs to cool itself down in hot conditions. A core temperature of 104 is considered to be heat stroke; at high temperatures, the heart and the brain can’t operate properly, and overheating can be fatal.
So the more tolerant you are of heat, the better your body gets at cooling itself. You sweat sooner and you sweat more. You may look more red-faced as your body increases blood flow to the skin.
My first sign that my heat training was working came when I was sitting in the sauna one day. The other women, who had been in there about as long as I had, were lightly glowing. I was pouring buckets of sweat, like I was sitting under an invisible showerhead. I was momentarily embarrassed, and then I realized—this means my heat training is working.
My mental gains were huge
Before attempting heat training, my only real thought about exercising in the heat was “ugh, this sucks.” But once I was exposing myself to heat on purpose, my mindset totally flipped.
During those early weeks, I found myself seeking out the heat. If I had the option of a morning run when it was 70 degrees, or an afternoon run when it was 80, I’d go for the afternoon run so I could rack up a better heat acclimation score.
Having a gadget put a number on my heat acclimation was highly motivating. (Both the Garmin watch and the Core 2 can do this, although a bit differently. I’ll say more about that later.) On the Core 2, I could see that each session bumped up my status a good bit, while each day spent in the air conditioning resulted in a slight drop. Looking at these graphs inspired me to schedule in more of those warm-weather runs.
I also just felt better about struggling. Whenever a run felt like a slog, there was an upside: I’m getting acclimated from this! In fact, the more the heat slowed me down, the more I knew I was benefiting. That’s a much better mindset than thinking “ugh, it’s hot and I’m slow.”
I pay more attention to heat safety now
I started off my heat training adventure by reviewing the Core 2, which isn’t necessary for heat training—you can just run more in the heat—but gave me a lot to think about. I interviewed Brian Maiorano, the coach liaison at Core, and learned a ton from him.
One “aha” moment was about the reason athletes are often encouraged to “pre-hydrate” rather than drinking to thirst, which I tended to ignore. Your body has a limit on how much water it can absorb through drinking each hour—about 1% of bodyweight—and you may well lose 2% or more per hour if you’re sweating a lot. For short workouts this isn’t a huge issue, but for long runs in the heat, those losses can add up. I started paying more attention to how I hydrated after learning that.
I also got to thinking more about the possibility of overheating. Normally I hide from the heat, so it’s not an issue. But now that I’m seeking out heat, I need to be more careful about safety. I had a bit of a wake-up call when writing up my review of the Core 2, particularly the part where I explained the “heat strain index” zones. There are four zones, and you generally want to be in zone 3 when heat training. Zone 4 is too high.
Writing that, I remembered an interval run I had done in the heat. I went back and checked my data from that run. I was pretty miserable through the whole workout, but toward the end I wasn’t able to keep up my intended pace at all. It turns out I had hit heat zone 4 at right around the time my pace went down the toilet. I’m a dummy, so I kept running. (“I’m almost done, I can get through this!” I kept telling myself.) From the data, I could see 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 cooldown run for a cooldown walk. The moment I dropped into zone 3 was almost exactly, to the second, the same moment I decided I was feeling OK enough to jog again.
Takeaway #1: I’m impressed that zone system seems really accurate. Takeaway #2: What the hell was I doing, continuing the workout when I was overheated and then insisting on getting in my total mileage for the day even though I knew I was too hot to run?? I’m glad I chose to drink water, stay in the shade, and walk rather than run, but in hindsight, I should have skipped all that. My car has air conditioning. Why didn’t I cut the run short and go sit in the car to cool down?
Since then, I’ve become better about safety. I pay more attention to how hot I’m feeling, and I now plan in cooling breaks if I’m going to be out for a while on a hot day. For example, for my first few 10-milers of the summer, I split them in half: five miles, then sit in the a/c for a while and have a snack and a drink, then go back out for the other five. That may not give me the exact same training stimulus as a continuous 10-mile run, but it’s a lot safer on the summer’s hottest days.
How the Garmin and the Core 2 helped (and didn’t)
As I mentioned, I have two gadgets that keep track of heat acclimation. One is the Garmin Forerunner 570, which has a built-in heat acclimation score.
The Garmin uses weather data to come up with a heat acclimation score after outdoor runs. (Here’s Garmin’s page on how that works.) It doesn’t know how hot I, personally, might be feeling, and it doesn’t account for indoor workouts at all. It’s pretty basic, but to be fair, its acclimation score tended to match up reasonably well with what I saw from the Core 2.
The other gadget is the Core 2 monitor, a more specialized device that retails for $295 and attaches to a chest strap. The big plus of the Core 2 is how detailed it can get with data from your run (as I described above) and with tracking how your heat tolerance builds and decays over time.
The biggest minus of the Core 2 is that I only get that data if I remember to wear my chest strap for every run, and if the Core 2 is properly functioning.
There are quite a few caveats on that last part. One is that the battery needs to be charged, and that’s in addition to making sure that the heart rate monitor and my running watch are both charged. Another is that the Core 2 has to be on. You can save battery by allowing the Core 2 to go into standby mode when it’s not being used, but that means you need to turn it on by shaking it before you start your run.
I definitely missed a lot of data by forgetting to turn it on. Several times I was a mile or so into a run, and thought “wow, I’m hot, I wonder what heat zone I’m in.” Then I’d scroll to the Core 2 screen and realize it wasn’t on at all. So I turn it on, and it starts collecting data at that point. Whenever this happens, for the next mile or so my core temperature on the device seems unrealistically low. So that’s two miles’ worth of data that I can’t totally trust.
One day, the device apparently malfunctioned. It was a sweltering day, and I was surprised to see that the device thought I was staying nice and cool. I ran another sweaty mile, and then collapsed on a bench in the shade. Checked the watch again. It still thought my core temperature hadn’t increased at all. Ultimately I had to ignore its data for the rest of that run, and then look up how to do a factory reset when I got home. It worked better after that, but I was disappointed that it had failed on me.
I also just straight up forgot my chest strap plenty of times. There’s no way to retroactively add a heat training session (besides a sauna session), so that data just didn’t get entered. The same is true of the Garmin, but it’s a lot harder to forget your running watch compared to a chest strap that, most of the time, I left hanging to dry in the shower.
Honestly, I think both gadgets were useful in the spring as I was first getting acclimated, but they aren’t all that useful in the summer. Every time I finish a run, my Garmin tells me that I’m 100% heat acclimated. Of course I am! It’s August and it’s 90 degrees out!
Similarly, I’ve forgotten to wear the Core 2 for my last few runs, but I also haven’t even checked the app lately. It’s hot today, it was hot yesterday, it will be hot tomorrow. I’m just not getting much out of any heat acclimation data when I know I’m heat acclimated.
What I’ve learned for the future
My experiment is only two-thirds over. The first phase was the process of becoming adapted to the heat this spring. That was successful, and both gadgets were helpful (the Core 2 being my favorite here). The second was running in the heat, which is going well, and the Garmin watch is the handier companion since it’s a bit more foolproof.
The third phase of the experiment will come when I see what happens when the weather begins to cool down. In theory, heat training gives athletes a boost in cool weather. (That said, the science on this has had mixed results, and heat training may not make a huge difference for cool-weather competition.)
I am not using the Core 2 often enough to keep its heat acclimation score up to date, but I do like that I can wear it during a run and get an instant readout of how much the heat seems to be affecting me at that moment. One use of this is to tell when you’re overheating; another is to tell whether, during a race or a cool-weather workout, you’re managing to actually stay cool enough that the heat isn’t screwing you over.
I’m looking forward to seeing what happens this fall. I’m thinking about doing an October race, maybe a 5K or maybe a half-marathon, and seeing how the drop in temperature affects how fast I can run.
And next year—who knows? I may or may not use a gadget to track my heat training, but I think I’d like to apply my mental gains to spring training, seeking out the heat so I can adapt better, and keeping a more positive attitude about those sluggish warm runs. I’ll also be more mindful of hydrating well and staying safe in the heat. So far, the whole idea of heat training has definitely helped me; without it, I’d probably still be doing short runs on the treadmill instead of planning another half-marathon-length trail run for next weekend.
