Exercise and Heat: How It Affects Us

by Angela (Oh She Glows) on July 9, 2009

More Green Monster stories have been updated over at Green Monster Movement!


I received my latest issue of Runner’s World in the mail this week and I was instantly intrigued by an article featured on the cover,

‘Turning Up The Heat: How exactly does hot weather affect runners?’


Given that many of us are now turning to the outdoors to workout I thought that talking about this topic was quite fitting! I know for myself I have been questioning lately whether I am getting enough water, wearing enough sunscreen, and replenishing enough electrolytes.

The University of Connecticut has a fun little room they like to call the environmental chamber. In this 10-by-12 foot bunker they can control the temperature, which is capable of reaching 110F/43C. The beauty of it is that they are able to control the conditions while monitoring things like heart rate, sweat loss, dehydration, etc.

The principal researchers, Professors Douglas Casa, PhD and Lawrence Armstrong PhD are two of the leading authorities on exercise in the heat. In addition, they are both lifetime runners, which makes their research all the more practical.

Here’s what the researchers did:

  • They monitored a runner in the environmental chamber running for 1 hour at an 8:30 min/mile pace on a treadmill. The room was a cool 53F/11.6C. The assessed the following factors: heart rate, rectal temperature, lactate, sweat loss, percent dehydrated, plasma volume.
  • The water amount he consumed in the first run (two sips) was the same amount of water that he could have in the hot run.
  • The next day, the same runner returned and they had him run for 1 hour at the same pace, however this time the room was 90F/32C!

They asked questions like: How hot would the runner get? How much sweat would he lose? How high would his heart rate get?

Before the runs:

  • They took a pre-run blood sample
  • A heart rate monitor was strapped on the runner’s chest
  • The runner is weighed on a scale
  • The runner gives a urine sample and inserts a wire/probe into his anus to obtain a steady reading of rectal temperature. They wire stays in his entire run (fun times!).
  • Every 20 minutes the runner breathes into a mouthpiece that assesses his oxygen consumption
  • After the 60-minute run, the runner gets a blood sample again to measure Lactate

The runner said the first run was a ‘breeze’ in the 53F conditions, however the next morning running in 90F with no water bottle was an entirely different ballgame! He became moody, testy, and irritable on his hour long run in the environmental chamber.

  • At the 20 minute marker, the runner gets 1/2 ounce of water- 1/2 ounce!!! He says, ‘I couldn’t even taste it going down.’ lol. At 50 minutes he got another sip of water.
  • Following the run, his blood sample was taken again, he gives a urine sample, and removes the rectal wire!

The Results                            53F                                           90F

Heart Rate                             158 bpm                                    175 bpm             

Rectal Temperature                101.98                                      103.46

Lactate                                   .978 mmol/liter                            4.04mmol/liter

Sweat Loss                                27.05 ounces                             54.10 ounces

Percent dehydrated                    1.3%                                         2.6%

Plasma Volume                           -.2%                                          -10.9%

During the 90F run, his temperature reaching 103.46, which is dangerously close to heat stroke which is said to occur at 104F. His lactate threshold climbed to over 4, which is supposedly when the leg muscles no longer function efficiently. His plasma volume reached 10%, which in combination with 2.6% dehydration, made his heart work a heck of a lot harder to push blood to his legs. 

I was quite interested in how much his heart rate increased! I know for myself, in warm weather my HR tends to be much higher.

The researchers concluded that if the runner had been on the treadmill for much longer, he may have suffered heatstroke.

What factors put you at risk for heatstroke?

  • Sleep loss (you aren’t sleeping properly)
  • Unusual fatigue
  • The sense that you are about to ‘come down’ with an illness
  • A long workout
  • a heat wave
  • extensive heat exposure (mowing the lawn earlier that day)
  • reduced sweating
  • fever or illness

Here is how you can play cool when running in the heat and reduce your risk!

  • Exercise during the coolest times of the day (before sunrise or after sunset)
  • Run in the shade- run on trail with trees
  • Wear light fabrics
  • Hydrate before, during, and after your exercise. Be careful though- over hydration can be just as harmful.
  • Exercise a shorter amount in hot weather
  • Workout at a lower intensity than you normally would
  • Workout with friends or let someone know where you are going when when you will return.


Today’s question:

Do you workout outdoors in the summer? What do you do? Do you pay any attention to the heat and how you will modify your exercise? Did anything in the study surprise you?


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{ 36 comments… read them below or add one }

Lindsay @ A Taste of Sparkle July 9, 2009 at 9:18 am

I just got my copy of this month’s issue and thought this study was really interesting. One of the things I thought was interesting was how in order to keep the controls in the study, he had to have the same amount of water at both temperatures – and he didn’t choose to have that much at 53 degrees! It shows just how important it is to hydrate, especially in these hot hot summer months.

Great point, I forgot to include this but will put it in!

I try to get my workouts in either in the morning before 12 p.m. or in the late afternoon once it cools down a bit. Running in hot, blazing sun is brutal. Another important thing I do is to always wear sunscreen.

Great post!



Courtney's Concoctions July 9, 2009 at 9:31 am

I change the way I workout in the summer. I take it easy and walk and do more yoga. Fall and spring are for running, winter is for snowshoeing and snowboarding. This way, I can exercise and eat seasonally!


Ellen July 9, 2009 at 9:40 am

I might have to go and buy runner’s world! I live in Texas and am training for my 3rd marathon, (my brother and I will be doing it together in October). Anyway, we have had record heat down here already and training was rough until I bought one of those hydration belts. It rocks! I try to run early in the morning, around 7am. Although, I may have to get up before dawn to do my really long runs. I was really surprised about the lactate. Thanks for sharing!


Julie July 9, 2009 at 9:47 am

I’ve done Half Ironmans last Summer and just last month in hot temps ranging from 88 to 97 degrees after training in very mild spring Oregon weather. The heat kills me. I tend to actually over hydrate during these hot events and need to learn to find a middle ground so my tummy doesn’t get so “sloshy”.
This study does not surprise me at all. It encourages me to do more “heated training” prior to my events – actually getting out DURING the hottest part of the day (doing it safely, though) so that my body isn’t so shocked come race day.


Susan July 9, 2009 at 9:50 am

I run outside year round, and it’s easy to see that I run slower but at the same effort when it’s hot. I drink TONS of water and try to avoid the hottest times of the day. As demonstrated by the 2007 Chicago Marathon, all the summer training doesn’t necessarily help when you have to go 26.2 in the heat!


Heather @ Health, Happiness, and Hope July 9, 2009 at 9:52 am

Awesome info Angela! As soon as summer rolls around, I’m always exercising outside. However, on days when it’s 90*F or more, I ALWAYS take it easy and only go for walks. I also drink a lot of Smartwater to keep me hydrated and refueled with electrolytes.



Jenn July 9, 2009 at 9:53 am

I hate running in the afternoon heat. I honestly don’t know how ANYBODY tolerates it. I get waaaay to hot. For that reason, in the summer I prefer to do my runs in the early morning before I go to work. If I can’t manage that, I’ll take my workout indoors to the gym. The only time you’ll find me doing any activity outside is when I have to do yardwork, which I’ll do until I work up a good sweat and then cool off by taking a dip in the pool.


Heather July 9, 2009 at 9:57 am

Good stuff! It gets pretty hot & humid here in the summer (Northwest Indiana) & we definitely plan our outdoor workouts better. We run in the morning (often around 530am) & hydrate better the night before if it’s going to be hotter out. Being out in the afternoon heat is just brutal some days & you start sweating the second you open the door!

A nice cool shower is the best way to cool off & then I’ll make a smoothie of sorts too.


Kristen July 9, 2009 at 10:03 am

I read that article too and it didn’t surprise me at all (although I also think it points out the importance of hydration even when it is cooler- he was still a little bit dehydrated after the first run).

Although I HATE the heat, I do still run outside in the summer. On really hot days, I do my best to run early in the morning or later in the evening once the sun starts to set, but often I end up doing my shorter runs during the hottest part of the day (which in the city tends to be around 4 or 5pm). I make sure to pay attention to hydration and I try my best to seek out the routes with as much shade as possible. Since I do my long runs on Sunday, if is going to be really hot, I definitely make sure to get up extra early to get it in before the heat comes.

Much like the previous commenter Julie, however, I do think there is some value to running during the hot part of the day if you think there is a possibility that you will be racing in the heat. You just have to be cautious and know when too hot is too hot!


Shelly July 9, 2009 at 10:07 am

Running in hotter weather has pros and cons for me. The pro is actually that I have exercise induced asthma, but when it’s really hot and humid, I don’t have to use my inhaler.
The cons are that sometimes in New Orleans, it is WAY too hot to run outside, period. We had a heat wave for about 3 weeks and I tried once or twice, always in the evening, and it was torture. It’s actually cooled down a little bit and I am so happy that I can actually get a good run in! I feel uncomfortable on a treadmill, and no other workout makes me feel as good or as worked out as a good run!
Symptom wise, when it is hot, I definitely notice my heart rate going higher than it would otherwise, and I feel a lot more fatigued than when it is cooler. I try to address this by drinking more water prior to a run, and running in a park where I can stop for water at several water fountains. In cooler weather, I hate stopping, but I make myself stop when it’s hot b/c I figure I need a sip of water even if I don’t think I do. The park also has a little workout circuit where you stop at stations and do some strength exercises. I find if I run to each station and do the exercises, it gives me a chance to lower my heart rate, which makes me feel better.


Aaron @ CrossBorderCravings July 9, 2009 at 10:10 am

What an interesting study! I wonder if the results would have been as extreme if he were allowed to rehydrate at appropriate amounts. I would think that would have accounted for a lot!

I definitely have to change my workout schedule in the heat of summer. I like to run outside around 5 pm when its cool, but in the summer heat I have to push it back to morning workouts (which I hate!). Either that, or I just head to the gym to run on the treadmill.


Jennifer @ His N' Her Health July 9, 2009 at 10:22 am

I run on the treadmill in the summer because it gets up to 110 F here. Even at 6 am it is almost 90! I have a really hard time doing physical activity in the heat. I have tried and I can’t handle it. Once while hiking my dad had to carry me out of the park because I got so overheated I could hardly walk.


Lori July 9, 2009 at 10:25 am

I definitely change how I workout in hot weather. I love to be outdoors, but I only do my intense workouts in the early morning when it it cool. If it is really humid, I also will not exercise as hard.

Lots of extra water, too!


Maggie July 9, 2009 at 10:27 am

As someone from South Louisiana, this is great info to live by! I can’t even begin to tell you how much a ten degree jump in temperature can affect a run.

Also, if you have had heat exhaustion or heat stroke in the past, you should know that all of the effects of hot weather are magnified. My dad had heat stroke 5 years ago and the weather takes a toll on his heart rate and dehydration waaay faster than it used to and than it should!


cowboyboot lady July 9, 2009 at 10:30 am

I quit the gym to save $ and have been running outdoors in 90+ degrees. I definitely notice my body straining itself and I find it more difficult to breathe. I normally run alone and take my cell phone with me just in case. I usually don’t hydrate during running, but I will start!


Barbara July 9, 2009 at 10:32 am

I usually run 1/2 marathons in winter only and then do A/C treadmill workouts during the summer. It’s going to be 106 with a heat index of 114 today, I mean…who wants to run in that?



Kelly July 9, 2009 at 10:36 am

Though not quite the same, how about running indoors on a treadmill when it’s super hot outside?

I’ve been working out to DVDs lately because it’s just so hot. We also do a lot of hikes on the weekends but running is out of the question. Not until it gets cooler.


Jenn Eats Nutritiously Now July 9, 2009 at 10:48 am

I always try to run in the cooler times of the day. If I can’t get in my run outside, I just bring it inside. I’m trying to get healthier– no need to risk it!


Susan July 9, 2009 at 10:48 am

Thanks for the info Angela!! Sort of what I expected, but it’s neat to see the numbers. Some of them are quite shocking!!

I never run in the heat. If I know it’s going to get hot, I try to get out in the morning. Biking is a little different, since it’s a breezier. But I try to never get out in anything warmer than 25 C. Anything higher is likely to make me feel sick after!


Cait (Cait's Plate) July 9, 2009 at 10:51 am

This is awesome advice! Thanks for all the info :)

I love that the summer nights stay light for longer so that I can manage to get a run in after it’s all cooled down. I’m always careful about running in the hot hot heat because I’ve heard too many dehydration horror stories!

Thanks again for sharing!


Low July 9, 2009 at 10:55 am

Thanks Ange! I keep my runs inside during the summer. It’s been between 95 – 105 here for the last month! So it’s still hot in the early mornings and late evenings. Once it starts cooling down a bit though I’ll definitely be taking it outside.


Lina July 9, 2009 at 11:03 am

Wow! Thanks for all the information!

I go running around town in the summer. It’s a nice 5 K run if I run around the town and the lake. I try to go really early in the morning so that it isn’t too hot since I feel lethargic in hot weather.

I don’t usually bring water with me when I run (bad, I know) but I’ll definitely start bringing it now that I’ve read that study!


katherine July 9, 2009 at 11:14 am

I definitely change the way I work out in the summer. I’m training for a marathon now and I’ll have to do all my training runs early in the morning or indoors b/c running when it’s super hot totally wipes me out! Bad for the body, and the soul!


allijag July 9, 2009 at 11:15 am

Great post! I know I feel the difference between running in the heat vs. cool – but it’s nice to see there is statistical data to back up the way I feel! Thanks for sharing!


Fitzalan July 9, 2009 at 11:15 am

“the rectal wire”…that in itself would make me moody, testy, and irritable!!

I do not allow myself to run outside if it is warmer than about 83 degrees. I have learned to recovery is just too much. Why ruin my body for 2 days for a little 40 minute run? During the summer I always try to run in the morning. The evenings are still humid around here typically to make it safe.

When I do run in the heat (anything over 70, I try to take precautions), I do the following:
–sunscreen (love the spray on sport sunscreen. Light, quick, not greasy and it works!)
–water bottle

Happiness Awaits


Help Meghan Run July 9, 2009 at 11:47 am

We had a group run last night at about 7, and it was still ~95*. :( We did about five miles and I was actually pretty good because I’m fairly used to it, but I make sure to drink plenty of water before I run and lots during the run, too. We stop A LOT for water breaks because it’s so damn hot. Great info!



MarathonVal July 9, 2009 at 11:47 am

My husband always teases me for being “temperature sensitive”, but it’s so true. I experienced the hottest and only Chicago Marathon of all time that got SHUT DOWN due to heat in 2007, when temps got over 100 degrees, so I know first hand how detrimental heat can be for runners. Sadly, one runner even died that day, and while it was inconclusive, most doctors speculated that it was at least in some way related to the heat.

Another great tip is to soak a towel or visor in water and freeze it, then wear during your run! :)


Kelly Turner July 9, 2009 at 11:51 am

I just watched a TV show on that where they ran the guy to near heat exhaustion. His core temp got a few degrees before heat stroke, but his legs gave out first and he collapsed. It was kind of funny though because he was narating the whole time, and then WHAM! hit the dirt.

I like to hike when its hot- it can be just as intense and the trees provide good shade


Jessica July 9, 2009 at 11:53 am

It’s not surprising, but that is why one should drink water! When I run outside and it is hot, I do feel a little light headed often, and like sweating bullets, but I wish I had water with me. That is why I want a camelbak for those purposes. I do like running on a hot night the best though. Say it is 80-90 degrees, and the sun has gone down behind the horizon, but there is still visibility. Unfortunately it doesn’t stay that hot at night in Nebraska,so I run during the day if I can.


aron July 9, 2009 at 12:23 pm

I try to get most of my runs done in the morning during the summer, since its usually 80+ outside after work. If I do run after work then I make sure to have water and other appropriate fuel and make sure to pay closer attention to my heart rate/breathing. Usually I have to slow it down, but after a few weeks running the heat, I can usually get semi-acclimated. I try to make sure to get any intense workouts/speedwork done when its cooler.


Lizzie July 9, 2009 at 12:29 pm

I love running in the summer (well, not extreme heat of course!) – I see myself sweat more and for some reason that makes me feel stronger. Plus I am not wearing 50 layers! :)


Rachel July 9, 2009 at 12:31 pm

Those differences are amazing! It’s nice to see it all laid out like that. I have a really hard time exercising in the heat – mostly ’cause i get so THIRSTY! So I mostly stick to mornings and late evenings if I do anything outside, and hit the gym for the rest.


Ally July 9, 2009 at 1:12 pm

I run outdoors on my long runs and try to go out super early (5:30 ish) and usually take a fuel belt with me, but, I stop at every water fountain (I run on a trail that has them about every 2.5 miles) and drink and then add some water to my water bottle. I’ve never gotten really overheated until last weekend when I ran a race that didn’t start until 8 am–it was only a 10k, but, they didn’t have nearly enough water stations (at mile 2 and 4.5!) and even though a lot of the race was shaded, a good portion wasn’t and it was 100F when we finished! YIKES!! I stated to get really sick to my stomach and got the “prickleys” or goosebumps which means I was headed downhill and fast. It was kind of scary. Note to self: bring your own water to all races. No matter what!! Not a fun lesson to learn, but, a lesson nonetheless.


Ruby July 9, 2009 at 2:00 pm

Wow great article! So funny – the runner from the article had about the same HR as I did! No wonder my hot-weather run was with an av HR of 175!


Andrea July 9, 2009 at 5:16 pm

I prefer to run outdoors rather than a treadmill. Here in AZ, even at 6am or 8pm the summers 100+ degrees. I definitely struggle more running outdoors in the summer and came to the conclusion that 2 miles is all that I should do outdoors to play it safe. This is a great article.


Salina July 9, 2009 at 8:53 pm

Wow! Great post Angela! I live in rural Australia, so we’re in the middle of winter and I’m pretty sure its 20C outside –beautiful day! Unfortunately, this means our Summers are HOT! 32C would be considered a pretty mild day, I’m not much of a morning exerciser so I usually have to wait until the sun goes down to do my workout.. It would be interesting to see what affect hummidity has on a workout.. I know that in Melbourne its easy for temps to soar past 40C, but they don’t have much hummidity like they do where I live, and i’m telling you, I would much prefer a 40C dry heat, than a 35C heat with 80% hummidity!


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