Why do I get Hot when I Exercise?

Stupid question right? So try and answer it. Think about why you get hot in as detailed a way as you can. Now compare it to what you read next…

We humans are a type of animal that generate our own heat through internal chemical reactions. Unlike some other animals (like lizards) we need to keep our temperature in a narrow range, otherwise bad things will happen to us. When our temperature rises too high we call it hyperthermia; when it gets to low we call it hypothermia. Our temperature rises as a result of exercise but obviously that’s not the only reason – our temperature can also rise through absorption of heat from the outside. But for the purpose of this article, I’m only going to deal with the internal aspect to heat production and dissipation. Sound like fun? Then read on brothers and sisters.

So you’ve all experienced it: running for a bus; sprinting up a hill on your bike; doing push-ups on the bow of your yacht. It isn’t rocket science – you exert yourself, you get hot. But why? It has to do with thermodynamics and, specifically, entropy. Oh no, here comes a flashback from when you were 16 years old… (or that old Flanders and Swan song – Ed)

What follows is a very simplistic explanation, but it should do the trick…

IMG_0188When energy goes from one state into another you lose some of that energy as heat. Here’s an analogy that actually makes sense: when a coal-fired power station turns chemical energy into electricity (by burning) not all of the chemical energy turns the turbines – some of it lost. Likewise, when the chemical energy in the food you eat is transformed into ATP (the molecule responsible for muscle contraction) the transformation process is not 100% efficient. Some of the chemical energy is lost as heat, or, we could call this entropy. Now here is where it gets more interesting:

We humans have several different ways of making ATP inside our muscles.  Some are more efficient than others. At low intensities, when we only require small amounts of ATP (like walking), we are most efficient. Let’s suppose we are 40% efficient in this instance. This will mean 60% of the chemical energy from your food is being lost as heat to the surrounding tissues in the body. When the demand for ATP gets higher (like running), the chemical conversion gets even less efficient. At a mildly intense jogging pace you are probably only 25% efficient at creating ATP, so 75% of the potential chemical energy is being turned into heat. When you are in an all out sprint, you are probably about 10% efficient – and so about 90% is going to heat. That’s a lot of entropy. And that is why the harder you work, the hotter you feel and the more you’ll sweat. I think I’ll explain about sweating at a later date…

Why you need to stay warm (but not too warm) when exercising

Some heat generation is beneficial to help speed up chemical reactions in the muscle. But excessive heat can adversely affect muscle contraction.

Chemical reactions in muscle cells (and within all your body) are controlled by enzymes. These chemical reactions are what allow muscles to contract. It’s a bit complicated, but basically an enzyme is a protein which speeds up a chemical reaction within your body. Forget about the enzymes you may have heard about in laundry detergents or digestion for a moment, every cell in the body has enzymes in them. We have specific enzymes for specific chemical reactions. These chemical reactions take place on a second by second basis inside our bodies – and – if they happened too slowly we would die. Quickly.

After a lull in fighting, GI Joe poses for a headshot.
Enzymes facilitate faster reactions – allowing all body cells to work as efficiently as possible. Heat itself can also make a reaction happen more quickly: drop a spoon full of sugar into a glass of cold water and see how long it takes to dissolve. Do the same thing in a glass of warm water and see the difference. It’s the same in our body – the hotter we get the faster chemical reactions happen in our bodies. Up to a point. The heat can get so high that the enzymes can “denature” (die). If you want to visualize what this looks like then think of an egg. Or specifically the white part. Egg white is pretty much all the proteins that go into the formation of a bird. When you heat egg white what happens? It turns opaque and gets tough. The proteins in your body can undergo the same process – they coagulate, change structure and lose any semblance of what they were intended for. Excess heat can cause enzymes to deform like a GI Joe held over an open flame. The plastic will bubble and twist and pretty soon it won’t look like a GI Joe anymore and it definitely won’t be able to fight Cobra. If your enzymes get too hot they will twist and deform and they won’t be able to perform their function either.

My next article will be about how our bodies keep our temperatures down when we exercise. This will be useful to know because the days are getting longer and hotter for us in the Northern hemisphere and you’re all going to be outside jogging up a storm this summer. Right? RIGHT?!?!?!

Please leave questions or comments below!

You can read all of Matthew’s series on fitness here

Article by Matthew Linsdell

April 13, 2012

Matt is a certified personal trainer and has a degree in Environmental Science. He calls himself an evidence-based trainer, because training is a field which is littered with well-disguised pseudoscience – his emphasis is always on teaching the biology behind exercise. He lives at the edge of the beautiful and expansive Gatineau Park in Quebec and works across the water in Ottawa, Ontario. If he’s not out walking his two pit bulls, you’ll find him doing press ups with insanely large weights on his back. Follow Matt on Twitter at @smartfitmatt.

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8 thoughts on “Why do I get Hot when I Exercise?”

  1. I get plenty hot when I exercise! It’s kind of interesting to know why. I am looking forward to you writing about sweating as I do that a lot too! My question is kind of the opposite to the topic of this piece. Why do I sometimes feel really cold after exercising? Sometimes as little as an hour after sometimes longer. Usually the harder the work out, the hotter I get, the more fatigued I am afterward, the colder I am.

    1. Linda, I’ll try to address this in the sweating article. But right now, I’m not sure.
      It could be that you are just experiencing the exaggerated effect that sweating has. Which is to cool the body. Or it could just be you notice how cold you are because of the contrast of how hot you also were. It would be interesting to record your internal temperature before and after a workout to see if there is in fact a real drop or if it is just a perceived difference.
      It is important for me to note that our temperature fluctuates throughout the day everyday. There is no such this as “regular” body temperature. 98.6 degrees Fahrenheit is a myth. An interesting one at that.

      1. “There is no such this as “regular” body temperature. 98.6 degrees Fahrenheit is a myth. An interesting one at that.”

        Sounds like another topic for you to write about!

  2. I actually have the same problem as Linda, I get way too hot when I exercise and then I get chilled later when my body cools down. It causes me a lot of problems as I now have a skin condition that itches like mad when I sweat. I also get tired of getting the chills later. I have to layer and strip down to shorts and a tank as I get hotter and then slowly put clothes back on as I cool down. I also get really sick if I over heat just for a few minutes and I wonder if Linda might actually have that same symptom as well. I also have the tendency to get muscle shakes after exercising, so I seriously wonder if I have some sort of metabolism issue. I’m also hypoglycemic.

    1. Laurie, you sound like your issues are way beyond my expertise and I would speak to a medical doctor about them. The only thing I am comfortable talking about is the layering. It is good that you layer and then shed but perhaps you should try not layering. Just wear less clothes at the start of the workout knowing you will warm up. I live in Canada and I run through the winter months. I’ll go outside way under dressed because I know I will heat up in 5 minutes to the point that I’ll be comfortable. It is worth a try. Also, getting too hot can cause a sick feeling and even vomiting. Not a feeling that inspires you to workout frequently. Talk to your doctor about these issues and report back if you find out anything interesting. I hope you sort this out. Linda is doing better everyday. I see her several times a week and she is getting fitter and fitter.

    2. Laurie – I don’t have any of those other issues. Really just chills after being hot working out. I have been paying attention since this post though and I find that it is the same feeling I get as when I am over tired or when I have spent a day on the heat outside (regardless of activity level) so maybe it is just overcompensation?
      Matt is my trainer and sees me two or three times a week and he is far too kind! I agree with him on the layering aspect. I prefer to be a little on the cold side when I start because if I overheat just a little before shedding the first layer or two I am miserable for the rest of the workout because it takes too long to reset my temperature. Hope you get things worked out!

  3. What happened to the follow article “how our bodies keep our temperatures down when we exercise”? I couldn’t find it anywhere?

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