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Day 1 – What Happens to your Body when Running in the Heat

Today starts my 365 day journey to runner enlightenment.  I think it’s only appropriate that I start this journey with what actually brought me to this point: my breakdown in Boston, all thanks to one hot ass day.  In an attempt to really understand what happened to my body and know how I can better deal with hot conditions next time, I am going to dedicate my first three entries to understanding how heat affects the body, knowing how to best run in the heat, and how to cope with heat related problems.

Today, my question will be:

What exactly happens to your body while running in the heat?

To anyone who has experienced both a hot race and cool race, you understand that physical activity becomes much more difficult as the temperature increases.  A 5K race on a hot summer afternoon is significantly more difficult than a 5K race on a crisp fall morning.  Why? What physiological processes are happening in your body for you to feel like you are working twice as hard but running twice as slow?

Well thanks to my handy dandy reference books, The Lore of Running and The Competitive Runner’s Handbook, I was able to find some scientific answers to this very complicated question.  But due to the fact that I am a Spanish teacher and not a Science teacher,  I am aiming to keep this in simple terms for those of us who can’t remember what exactly adenosine triposphate is.

When we begin to exercise,  the amount of blood flow to the muscles is increased.  As it passes through the muscles, the blood is heated and then heat is distributed throughout the body, and in particular to the skin.  As the temperature increases, the body must determine whether to pump more blood to the muscles (maintaining intensity) or to increase blood flow to the skin (cooling the body).  According to Dr. Noakes, the body will always increase blood flow to the muscles which takes away from its ability to cool the skin (meaning you get hotter).  In addition, the heart begins to beat faster in an effort to keep up with the dual demands.  Simply stated: there is a tug of war going on within the  body – cooling the skin and fueling the muscles.

So if we understand why the body is heating up (not enough blood to cool the skin because it’s too busy keeping the muscles going), how then is heat dissipated and what factors hinder this process?

Heat loss can occur four different ways (think back to elementary school science class): conduction, convection, radiation, and evaporation.  With conduction, heat is transferred to anything it touches that is cooler.  For example, ice and cold water conduct heat away from the skin because they are cooler than what they are touching.  With convection, as the air around our bodies warms, it moves up and is replaced with cooler air.  However, if the air is very hot (as it was in Boston), conduction and convection can cause the body to heat up instead of cool off (there is no cooler air to move in and replace the hot air).

The last two ways of dissipating heat are radiation and evaporation.  Radiation works basically by transferring heat to any surrounding object whose surface temperature is lower than the skin’s temperature.   If you are going for a morning run before the sun comes out to heat the roads, your body will radiate heat towards the cooler surface.  But do that same run at 3:00 in the afternoon and radiation will work the other way.  The asphalt has been heated and instead of you radiating heat to the road, it is radiating heat to you.

And finally, the most effective way to get rid of heat: sweat.  Sweat is able to absorb the body’s heat by conduction, and then it evaporates into the air which allows the body to cool even more.  When it is humid outside, however, the evaporation process doesn’t work so well.  Think about it:  the air is already full of moisture and hence can not absorb sweat too.  Therefore, us runners are left looking like we just jumped into a pool while we feel like falling over into one.

Sweating is by far the most important thermoregulatory response to heat.  But what if we sweat too much?  If the body loses too much water, it becomes dehydrated, which in turn throws all of the cooling mechanisms out of whack.  Body cells work inefficiently, sweating decreases, heart rate and body temperature increase, blood volume decreases, and less blood is available to circulate oxygen and glucose through the body.  Here’s an interesting fact:  for every 2.2 pounds of water lost, your heartbeat will increase about 8 beats per minute.  That’s a lot!

If I had to take this information and apply it to determine what went wrong in Boston, I would say the following:  I started running too fast and my blood was too busy keeping my legs going instead of cooling my skin.  In addition, it was already hot outside so the roads were radiating heat towards me, instead of the other way around.  Humidity wasn’t really a factor so I’m pretty sure my sweat did just fine evaporating.  However, I did a poor job of replacing all of those liquids I was losing through sweat, which in turn led me to become dehydrated.  As I became dehydrated, my heartbeat increased (maybe my 8 beats or more?), and the tug of war became too much.  My body decided that it had had enough and it told me no more.

So there you have it.  My semi-scientific explanation of what happens to the body when running in the heat.  Day 1, check.  Only 364 more to go.

Happy Trails and Happy Running,



And a random picture of my lovely kinesio tape. I think kinesio tape will be another research question…

2 Comments Post a comment
  1. Kim #

    Good information, and easy to understand – thanks! I find it was harder to run when it’s hot.

    May 19, 2012
    • Thanks Kim! Running in the heat is so not fun, but I guess sometimes it’s unavoidable. Today I’m writing about how to deal with the heat… off to research I go!

      May 20, 2012

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