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Thermoregulation and Running: A Cool Idea

Posted by on Jul 14, 2012 | 29 Comments

The Evil Moisture Wicking Clothing?

In a previous post, I theorized about the role a moisture-wicking shirt played in three crappy runs over the last seven months or so. After reading your comments and doing a little research, my “ah-ha’ moment wasn’t quite so revolutionary. The conclusion is pretty obvious:

Moisture wicking clothing does more harm than good for runners in hot weather.

The reasoning is simple- the mechanism of drawing moisture away from the skin thwarts the process of evaporative cooling of sweat. In other words, the wicking of the moisture means the sweat evaporates on the surface of the shirt, not your skin, so we’re robbing our body of one of the primary cooling mechanisms used to reduce our core body temperature.

The video from Sports Science explains the idea nicely, though I seriously question whether the X Bionic fabric would be as effective as bare skin.

Indeed, the marketing material from several moisture wicking companies confirms this. They are very guarded about saying the fabric cools you down in heat. Instead, they tend to use statements like “the fabric makes you feel more comfortable.” Indeed, moisture wicking fabrics are great… when you’re not generating a ton of heat via exercise.

My new rule of thumb– I’m only using moisture-wicking fabric in cold weather or in warmer weather when my activity level is low enough where body temps can be maintained without the sweating mechanism.  If I am running in hot weather, I’m going with white cotton or no shirt at all.

Sidebar- the shirtless option may also have another benefit- prevention of internal cancers. Check out this article, and note the citation from Dr. Gordy Ainsleigh. We did a run with Gordy last week and he was, of course, shirtless.

Thermoregulation: My Missing Link

Over the last few days, I’ve been experimenting with perceived body temperature and how it makes me feel. I believe my problems at Bighorn, the disastrous Boulder to Nederland run, and Across the Years were all caused by overheating.

During each of those events, I knew heat was the problem… indirectly. I assumed I wasn’t drinking enough (dehydration) or I wasn’t taking enough electrolyte supplementation (hyponatremia). These are the two often-quoted causes of the symptoms I was experiencing. In fact, most runners recommended either drinking more or taking more electrolytes.

The feelings I was having weren’t quite right, though. In training, I have purposely run to a point of dehydration. Early in my running “career”, I also experienced the early signs of hyponatremia. Neither of these feelings were quite the same as I experienced in these three instances. There was some other variable I was missing.

That variable was a rising core body temperature.

Generally I’m a pretty good hot weather runner. I can tolerate heat far better than cold. In the humid Midwest, I used to run on the hottest of days with no issues at all. I didn’t experience this problem until I started running in hot, dry weather… and then only when wearing moisture-wicking shirts.

As I explained previously, the moisture wicking material apparently caused my core temperature to rise faster than my sweating mechanism (and passive heat-dissipation methods) could get rid of the excess. The result was extreme fatigue, dizziness, and severe cramping, all of which can be explained with Noakes’ “central governor” theory- my body was fighting me to stop moving to prevent, well, death.

In all three cases, I didn’t have signs of dehydration or hyponatremia. As such, drinking more or consuming electrolytes had no effect. What did help, however, was getting out of the sun and stopping activity. THAT allowed my core temperature to cool.

Some basic experiments seem to support this theory. I did two runs up a mountain. The mountain had several “micro-climates” of various combinations of sun exposure and wind. The goal was to try to replicate the feelings of fatugue, dizziness, and cramping without having to run a crazy long distance. I accomplished this by running at a strenuous pace up the mountain. I made sure I was adaquately hydrated and electrolytes were supplemented before the run to help rule those out as confounding variables. I carried one water bottle and drank to thirst throughout the run, which equaled about 12 ounces per hour.

The Results

I was able to mimic the exact same feelings I had in the three disastrous runs, though to a lesser degree. The symptoms were worse in sun-exposed areas with no wind, improved in the sun-exposed areas with wind and shaded areas with no wind, and completely disappeared in shaded areas with wind.

I was shocked the results were so clear. Previously, I was at a complete loss as to the cause of the symptoms because I didn’t consider body temperature to be a variable. It was never an issue in years of prior running, even when air temps were very high.

As soon as I recognized body temperature as a variable, the correlations became obvious. If sweat isn’t cooling me down, performance suffers. A lot.

The Solution

The solutions are obvious. I have to work on methods to stay cool. Here are the steps I will take:

  • Train in heat more often. Theoretically, this will help make the body’s thermoregulation system more efficient. At the very least, it will help train me to recognize the early signs of overheating.
  • In hot, dry weather, go shirtless. This will maximize evaporative cooling.
  • If there’s a lot of sun exposure, wear white cotton shirts. The white cotton will absorb less heat than my slightly tanned skin. The cotton, when saturated with sweat, will allow evaporative cooling via conduction. I don’t think it is as efficient as bare skin, but it is far better than the moisture-wicking materials I’ve used previously. Also, I’ve run in cotton in the same hot, dry environments with no issues at all.
  • During a race, take advantage of cold water (from aid stations or streams) and ice/snow by wrapping it in a bandana and placing it around my neck, dumping it over my head, or some variation of submerging myself. This will cool the body via conduction.
  • If all else fails, slow down. Since movement generates heat, a lack of movement will cool the body quickly. I won’t be so stubborn with stopping in the future.

I’m very confident going into my next two hot, dry races (Grand Mesa 100 in western Colorado and the Trans-Rockies stage race across Colorado). Both will feature hot temps and a lot of sun exposure. By implementing these solutions, I should see a significant boost in performance. At the very least, I’ll be familiar with body temp as a potential problem-causing variable. This should help me regulate both hydration and electrolyte levels, too, because I won’t erroneously blame them for issues caused by overheating.

In essence, we have to start treating thermoregulation as a separate variable that’s not necessarily controlled by drinking more or popping more salt tabs.

I know a few of my readers have been experimenting with ditching moisture-wicking clothing in hot, dry weather. Any feedback yet? What about thermoregulation? Does anyone have any other useful tips?




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  1. Glenn
    July 17, 2012

    I have been following the recent thread of thermoregulation blog posts and I haven’t seen anyone post about artificial sweat, i.e. water in a spray bottle. Since I’ve never ran or seen an ultra, I don’t know if this has been employed.

    My point is, if you’re running in an environment that is making your sweat evaporate before it can cool, why not employ a spritz of water now and then replace the rapidly evaporating sweat. Has anyone ever given this a shot?

  2. Brian G
    July 15, 2012

    So I geeked out after reading your first article and bought myself a microweight merino wool short sleeve t-shirt to test this theory out a bit in my weekend run. Overall the results seemed to be fairly positive, at least more positive than I would have thought running in a *wool* shirt on a warm, sunny, dry day in Colorado.

    I bought it a size smaller than I usually wear so it would fit tighter against my skin. I had to buy it in a darker, olive green color since that was the only color available in the local REI. Temps ranged from 65 deg F to 80 deg F. I ran in almost the same sun/shade/wind conditions you state in this article.

    I felt very comfortable throughout the entire run. Near the end I repeatedly took my shirt off and put it back on to see how I felt. I felt slight worse with the shirt off because the direct sunlight really felt hot on my skin.

    I’ll be wearing this shirt (though maybe a lighter color if I can find one) for now on on my LSD runs.

  3. Kyle Kranz
    July 15, 2012

    You also have to be conscious of shirt color and fit. I’ve read that a tight white shirt will do a better job of cooling than a loose one. Also a loose black shirt does a better job than a tight black shirt if there is wind since the black will actually absorb your body heat and it will be taken away with the wind. A loose white shirt will simply reflect and trap your own body heat inside the shirt against your skin. Or so I’ve read.

    • Dave Goulette
      July 15, 2012

      Good point bringing up the issue of how loose the fit is. I noticed that some of the Raramuri runners like Arnulfo Quimare wear loose, flowing shirts. As far as I understand, they live in fairly high altitude arid climate. Does anybody know what their garments are made of?

      • Jason
        July 17, 2012

        I suspect their shirts work a little bit like the black robes in the desert… like a thermal chimney. The sun heats the fabric, which creates updraft in the shirt. That thermal updraft helps cool the body via convection and enhances sweat evaporation.

  4. ElyDave
    July 15, 2012

    I’m going to be a dissenter here, at least until I’ve thought about it some more. I don’t entirely buy the argument both from practical experience and from being an engineer.

    I think you’ve not taken in all the variables, specifically internal cooling. I know that going shirtless on the turbo trainer does not confer any cooling advantage, the thing that seems to have a big effect here is the intake of cool water during a session, which has a demonstrated effect on my HR for a steady state session. Of course here I don’t have forced evaporation from movement through the air or from wind.

    Take a reasonably close fitting wicking shirt or cotton t-shirt that is sticking to the body with sweat, I can’t see either of them being a significant insulator, so even if moisture is being wicked away from the body and evaporating at the surface of the shirt, their should be a cooling effect.

    I do still run shirtless occaisionally though just to get a bit of sun on my back.

  5. Dave Goulette
    July 15, 2012

    Jason, I think you are very likely correct in your observations. I would like to try to explain a bit of the physics going on here as best I understand it. Please, anybody, let me know if you think I get anything wrong here. First, only the surface molecules of water evaporate and they evaporate because of an increase in kinetic energy. There are two main ways to increase the kinetic energy of surface molecules: increase heat and/or air flow. Only the surface layer of water evaporates, and when it does, the vaporization process causes a lowering of the average temperature of the liquid water below the surface layer. Since your skin is below the lowest layer of water, the overall decrease in water temperature causes heat to be drawn from your skin. Then the surface water heats again, evaporates, cools the lower levels of water, etc. etc. The cycle repeats. The process is called evaporative cooling. But air movement at the surface ALSO has the effect of increasing kinetic energy of the surface layer. Thus if there is air movement at the surface, the surface layer evaporates at a lower temperature and thus the evaporation cycle happens more rapidly. Increasing the surface area of water also increases the rate of evaporation and lowers the amount of water below the surface (which I think is a key factor for our purposes).
    Now my conjecture: I think that moisture wicking fabrics may be moving the sweat out to a larger surface area. This causes the sweat to evaporate quickly. Also this movement of sweat causes you to have a smaller volume of sweat BELOW the evaporating surface. This will lower the effect of evaporative cooling. Also the sweat is not evaporating directly over the point where your skin secreted the sweat and hence we are not losing energy directly from the places our body wants us to. When you wear cotton your sweat stays roughly over the point of origin. Thus the volume of sweat has a smaller surface area and larger volume below the surface. This will lengthen the cooling effect and it will occur directly over the place where you sweat which is where you need to dissipate the heat. All of this is more of an issue in dry climates where evaporation happens more quickly. We probably want to slow down evaporation to regulate body temp as much as possible.

    • ElyDave
      July 15, 2012

      Dave, I think you’ve got part of it right, that if the water has a larger surfaces area, potentially (with no other limiting factors) it could evaporate quicker. But where you argument falls down is that more water evaporating quicker equals greater cooling overall, so the remaining sweat would be cooled to a lower temperature which would be conducted to your skin.

      The argument of a larger surface area from a wicking fabric also seems a but far fetched. Think about how much thicker it would need to be than cotton for that to be true. One of the arguments put forward for wicking fabrics is the lighter weight, which I would also speculate translates to thinner.

      • David Goulette
        July 16, 2012

        Hi Ely. Thanks for the discussion. You state, “more water evaporating quicker equals greater cooling overall.” But there is not “more water”. There is the same amount of sweat evaporating faster. And my speculation (which would need experimentation) is that you don’t get as much of the sustained cooling effect from evaporative cooling since the sweat is gone so quickly.

        To your second point: are you saying then, that wicking fabrics allow you to evaporate sweat at the SAME rate as when you have no shirt on? I was under the impression that the wicking fabric kept you MORE dry than with no shirt partly because it moved the moisture away from your body (My speculations were based on this assumtion). This also leads me to speculate that we lose the evaporative cooling effect because our skin is not as wet as it would be in cotton or without a shirt. We need skin to sweat contact to conduct heat away from the body. I also thought that sweat evaporates faster with wicking material. And if this is true then it must be because the surface area of the sweat exposed to the surrounding air is larger. It seems that the other factors in evaporation rate would be nearly the same regardless of type of shirt.

        • ElyDave
          July 16, 2012

          Hey Dave
          I read an Einstein quote today, to paraphrase “you need to reduce any problem to it’s simplest, and no simpler”.

          Not wanting to hijack Jason’s blog, but I think you are oversimplifying the problem and not considering the entire model.

          To me the model/system is Body-shirt-evaporation, or body-evaporation. In the body-evaporation model you have the direct connection with evaporation at body surface. Body surface area can be estimated fairly well as can evaporative loading and hence cooling. Now the water can only evaporate at the rate at which energy can be supplied to it, by a combination of heat from the body and forced evaporation from the passage of unsaturated air (at whatever temperature).

          Let’s take the second system, body-shirt-evaporation. If we assume the shirt is tight fitting, then we have the same heat input from the body, and also the same unsaturated air for forced evaporation. The only thing different can be the surface area, so unless the shirt weave is massively increasing the surface area, I think that effect (in context of the whole body surface area) would be quite small. If the surface area is bigger, then evaporation of the water may be quicker, but the water supply is limited. The effect of this would be that teh water that is not evaporated is cooled more and would conduct a lower temperature back to the skin, therefore having a greater cooling effect per area. If the evaporation is greater than water supply, then the skin surface should be cooled pretty well and this may actually stop or slow sweating. If the water supply is greater, then cooling will continue, and again, the extra cooling may even reduce the sweating. I know from experience that if I wear a close fitting base layer on the bike with no windproof, on a cool but dry day or a warmer one with a lot of wind, I can actually end up feeling colder as I get much greater sweat evaporation rate.

          The thing that I could see affecting this is the fit of the shirt, a looser fitting shirt would uncouple the shirt-evaporation system, but would also then limit the wicking (with less surface contact).

          Hope that’s not too long winded

          • David Goulette
            July 17, 2012

            Thanks for the thoughts! I do see what you are saying about the fit of the clothes. It is likely an important factor. What, then, do you think about the Raramuri clothing that is loose? (See Arnulfo Quimare) I wonder if they wear anything under the running shirt. (BTW they look a bit like Jerry Seinfeld in the pirate shirt!)

  6. mark lofquist
    July 15, 2012

    convection, radiation, convection. these are three ways heat transfers. convection is wind blowing on your skin. wind blowing on a shirt that’s ‘coupled’ to your skin is less effective because that coupling mechanism cannot be lossless. buuuuuut, there’s a reduction in radiation from the sun, so that component of additive heat is reduced.

    so it’s very different in direct sunlight, cover up!

    in electronics we’d add thermal pastes, heat sinks, etc. each added component has a heat transfer coefficient, a coupling loss measurement, all pretty measurable. i’m mebarrassed to say i’ve never applied it to people and even more embarrassed that i’ve never read someone doing the math of Watts/(m^2K) on human skin versus human skin coupled to different fabrics.

    lesson learned here, m^2 is a number easily changed! heat sinks on a silicon chips, cooling fins on amplifiers, fins on a radiator… we need shirts with an extra dimension like squiggly ribbons that leaves our skin just a little bit would overcome lossy coupling and reduce radiation and increase convective cooling via increased m^s. lol, even tassles would help!

  7. Rob Y
    July 15, 2012

    IMHO, not all tech shirts are created equal. From my own anicdotal experience I’ve come to find that I do best running in very thin, silk weight tech shirts. My favorites for years have been the Patagonia line of silk weight shirts that have built in SPF. I for one feel so much cooler running in these sun shirts than even running shirtless or other, heavier technical shirts. I’ve used cotton for years in my early days and won’t ever go back. Cotton is inferior overall, at least in my experience, over a wide range of conditions. The real key to adequate cooling, as you’ve discovered, is to externally cool as well as internally. That means soaking your shirt and head when you have the opportunity; not just drinking to cool. In full disclosure I will say that I do still get a Patagonia Pro discount and so have had the opportunity to test a lot of technical shirts over the years. For hot conditions, both dry and humid, thin, SPF clothing really works for me. And I’m not a hot weather kind of guy, I prefer the freezing cold to intense heat. Too hot blooded! Regardless, I got through the Badwater Ultramarathon pretty well wearing thin, high SPF clothing and constantly cooling externally. Can’t imagine trying to run this extremely hot race in cotton! Doh!

  8. edh
    July 15, 2012

    My biggest problem with this is the chaffing with cotton. I switched to a technical shirt and have zero chaffing issues now. Now I need to go back to cotton to stay cool? Ugh.

  9. Ben lancaster
    July 15, 2012

    I could see how there could be a situation whereby if you sweat a lot, which takes a while for the desired amount of energy to transfer to the sweat to cause evaporation, then wicking some of that away would help the remaining sweat evaporate quicker.

    However for someone that doesn’t sweat a lot removing this layer of moisture through a mechanical action rather than an energy transfer will obviously not leave enough behind to perform it’s purpose.

    I have quite the layer of fur on me, which I generally blame for overheating, but slowly my body has got better at producing sweat (I used to rarely sweat, just get hotter and hotter) Now I see the hair pulling some moisture away from the skin, but not aggressively, which in turn creates a greater surface area for the sweat to absorb enough energy to reach evaporation point, and to have more chance of catching air currents to carry away this hot sweat.

    Bear in mind it’s a long time since I took a physics class 😉

  10. Barefoot Tyler
    July 15, 2012

    Today I did another run in a cotton tshirt with the sleeves cut off. Of course it was cool and rainy again. I would like to report that I did not have any problems with chafing. I did, however, lube my nips and arm pit areas(where my arms rub), as well as my legs which I usually do. I feel like as long as my shoulders are exposed I feel a lot cooler regardless of shirt. I still have to experiment more with the cotton shirt. I did like how it separated, instead of stuck, to my body. The sweat cooled when separated. I then took my hand and pressed my cool sweat against my body, which felt good.

  11. Franklin Chen
    July 14, 2012

    Very interesting speculations! I have run in synthetic moisture-wicking shirts for over a decade now, and I myself quickly noticed that they don’t help me when it’s warmer (although they are great when running in cold). I don’t have a “solution” as such, other than that I expose more skin to air when it’s warmer; I do this by wearing a singlet rather than a T-shirt, and for warm hiking I’ve taken to wearing button-up shirts that do not cling to my body but leave air in between.

  12. Seamus Foy
    July 14, 2012

    I just noticed that there is a space in my hydration pack where I could fit some ice packs. The back is one of the major areas where we release heat, so I’ll see if that helps. It won’t be a perfect comparison, but I can compare heart rate from workouts with and without the ice packs.

    I also read about a product called BEX Runner, which is supposed to cool from your hands. It sounds ridiculous, but who knows. I’m gonna do a diy version and run with ice bags in my hands. We’ll see!

  13. Matt
    July 14, 2012

    I’m really confused by the Gordy Ainsleigh article about the sun. I thought that we were supposed to cover ourselves with clothing and sunscreen to prevent sunburn/skin cancer due to sun exposure? Is this not true, are we supposed to expose ourselves directly to the sun and risk sunburn? I don’t understand, someone please explain!

  14. Joel Schafer
    July 14, 2012

    If one is wearing a sweat-soaked technical shirt that is plastered to your skin, and the wicking shirt is continually cooling due to evaporation, why would that not cool your skin as well? It seems like the same amount of heat would be leaving your body, provided the shirt isn’t actually an insulator, which it’s not.

    Or alternatively, if I’m wearing a soaked cotton shirt, how is water evaporating from my skin if it’s covered up by the shirt? It still seems like the water would still have to diffuse through the material and evaporate from the cotton shirt’s surface.

    One might imagine a shirt that pulled the moisture to the surface would have more effective evaporation than on that is more absorbant like cotton which holds on to the moisture.

    In any case, if it’s a very humid day evaporation is not going to be that effective for cooling anyway because the air is so saturated.

    I’m just speculating wildly here, but that’s my initial impression.

    • Jason
      July 14, 2012

      In my experience, a sweat-soaked tech shirt cools as well as a sweat-soaked cotton shirt. I would hypothesize this is because the moisture is a good conductor of heat. As the surface of the shirt cools due to evaporation, heat is drawn from the skin.

      Agreed about the humidity.

      • Chuck
        July 16, 2012

        I think the one fact I haven’t heard mentioned yet is the Wetbulb v. Drybulb temperatures. Wetbulb is the temperature that evaporative water can cool to. This is why a water based cooling tower is better than a dry ac condenser (in the HVAC industry).

        Sweating can only cool your skin surface down to the wetbulb temperature at best. If your sweating less or working harder it wont get there. A water soaked medium that is holding water and evaporating should get closer to the wetbulb then water just evaporating from a dry shirt.

        I think the problem with the moisture wicking clothing is that it evaporates so quickly and the shirt stays mostly dry it doesn’t approach the wetbulb temperature nearly at all. This can keep your skin temperature 15-20 degrees warmer in hot humid climates and 30+ deg warmer in hot dry climates.

        I don’t know if it’s true, but I would think a soaked shirt holding water and evaporating may actually cool you more than shirtless. This would provide a medium to get close to the wetbulb and stay there. Allowing more heat transfer from your body. Shirtless would allow sweat to fall from your body and not cool you as well. I think the difference would be minute, but my vote is soaked shirt.

  15. Adam
    July 14, 2012

    This makes sense to me. I never had any problem w/ my cotton shirts, but of course, like a lemming, I started wearing moisture wicking clothing exclusively because the marketing told me so. It seems clear now that synthetics are superior when staying warm is the issue, especially if there is rain and river crossing involved (although even there, shirtless may be better, so you don’t have an extra layer of wet conductive material between yourself and the air), but probably not for staying cool. My primary concern w/ shirtless (other then mosquitos and branches), is that I never feel like covering myself in sunscreen. But I guess if it’s between that and overheating, shirtless is the way to go.

  16. Seamus Foy
    July 14, 2012

    Thanks for writing this, Jason! Hydration and heat management seem to be my breaking point in running, and I don’t think I can run 100, or even a fast 50, until I get it figured out.

    I have a crazy high sweat rate, in the neighborhood of 8-9lbs. per hour on a hot day, so I’m on a mission to figure out how to manage it. I’m gonna try to come up with ways to use ice or ice packs during a run, as I think I can only reduce sweating by reducing body heat.

    Your writing about this topic has helped me think about this problem.

  17. Paul Wallis
    July 14, 2012

    I’m having a hard enough issue with attempting to twart the no barefoot running rule with the ITU, I can’t see them recanting on the required shirt rule as well. Although the effectiveness of running without a shirt on a regular basis may be a lot of fun to test out I can’t see it gaining in popularity with those of the opposite gender unfortunately. Perhaps it’s just as well, as I for one would find it difficult to concentrate on running…

  18. John White
    July 14, 2012

    I think we all fell for marketing. The advertising implication is that wearing an extra layer can make you cooler in hot weather. (Jason, you noted they are careful about not saying that outright…) It seems to me that’s a pretty bold claim and we should have been more skeptical. Extraordinary claims require extraordinary proof.

    I had a very comfortable shirtless run in hot (for here) weather the other day. Will keep exploring… One side benefit: an even tan!

  19. mark lofquist
    July 14, 2012

    i run where dew point (and that’s the magic #!!**) is close to or past 70deg F frequently this summer and NO shirt is definitely the answer.