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An Open Letter to Race Directors: Please Stop Distributing Tech Shirts!

Posted by on May 18, 2013 | 16 Comments

Dear race director friends:

I understand you have undertaken a thankless job. You put in countless hours to assure your event is well-organized. I also understand you receive a slew of dumbass requests. Hopefully some of you will consider this request even though it directly contradicts the accepted logic most runners ascribe to.

Please stop distributing moisture-wicking tech shirts.

Why you ask?

Quite simply, they’re endangering the lives of runners that falsely believe their clothing will help keep them cool in hot weather. Many runners believe the tech fabric’s ability to transport sweat away from the skin helps cool the underlying skin.

In reality, it robs their bodies of the evaporative cooling effect, thus raising their body temperature instead of lowering it. Sweat evaporating off the surface of the shirt doesn’t cool the skin under the shirt with nearly as much efficiency as naked skin.

In short, most runners don’t understand how their body thermoregulates.

To combat this overheating, many runners will attempt to drink more water or sports drink which increases the danger of hyponatremia.

I touched on the topic in this post here and here last year, and is a major topic in the Squirrel Wipe trail and ultra book.

I’ve done several reviews of the literature to find any empirical evidence that moisture-wicking fabric actually lowers core body temperature. Not surprisingly, none apparently exists.

There are studies measuring perceived comfort where tech clothing is rated as more comfortable than natural fibers like cotton, but the research is silent on the issue of thermoregulation. Yet clothing manufacturers continue to churn out moisture-wicking clothing with the implied or expressed promise it will “keep you cool.”

In reality, moisture-wicking clothing was developed for low-intensity cold weather activities that required frequent stops (i.e., hiking.) For that purpose, it works wonderfully. It also works well for areas that remain covered, thus preventing air circulation (think socks and underwear.) For hot-weather exercise, it’s flat-out dangerous. Not only will hyperthermia reduce performance, it can lead to heat stroke and eventual death.

Race directors can help correct this deception by refraining from distributing moisture-wicking clothing, especially in hot-weather races. Please stop contributing to the idea that this clothing will keep you cool. You could potentially save the lives of your fellow runners.

Thank you for your time, I know it is valuable.

Appreciatively yours,

-Jason Robillard



Runner friends: Please help me spread this important message and forward this post to any race directors you may know. 😉

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  1. Barefoot Running University » How to Stay Cool when Running: A Lesson in Human Thermoregulation During Exercise
    May 19, 2013

    […] do we stay cool when running in hot weather? In my last post, I challenged race directors to reconsider the growing practice of including moisture-wicking technic… That resulted in a slew of questions and comments ranging from skepticism to recommendations on […]

  2. Mirko
    May 19, 2013

    I would like to say that not all the tech shirts are the same.
    I own a lot of material of X-Bionic, the brand that you linked in your video, that say that with their staff they achieve the goal to thermoregulate the body.
    I can say that they keep what they promise, their stuff help the body to keep the right temperature of 98°F (37°C), they help warm you when it’s cold and they keep you cool when it’s hot.
    For hot and sunny weather, they have a line called “Fennec” that really works well when it’s very hot.
    I have to say I’m a big fan of this brand, and I discovered it after watching your blog and the video you linked. I also have to say it’s expensive, buy I think it’s worth the money. Better to have one good shirt, than 10 bad shirts.
    And I think that that material is better than cotton, because cotton becomes all wet when you’re sweating. The x-bionic shirt keep just a small amount of sweat on the skin when you need to cool, and removes the excess of sweat. At the end of the run, the t-shirt is wet but you’re skin is almost dry.Hard to believe, but it’s true!
    This brand has different models for hot weather, cool weather, cold weather and it works very well.
    But I warn you, it’s very expensive, the only other brand that costs so much is the Gore (I never tried it, I heard it’s good. But I heard x-bionic is better for transpirancy and treatment of sweat).

  3. HeadDoc
    May 19, 2013

    The lightweight merino t-shirts from Smartwool, Ibex, etc. seem to be the best of both worlds…

  4. thinnmann
    May 18, 2013

    When I saw the headline of this post in my soon-to-be-defunct Google Reader, I thought you were going to go for 1.) they are bad for the environment, and 2.) I have so many tech shirts now I can’t friggin’ close the drawer. I wish that was what you were going for! So, Dear Race Directors: fees keep going up, so offer me a $10 savings for no shirt, because I can’t friggin’ close my drawer! And they are not even good for checking the oil or washing the car, like cotton/poly crappy race shirts are. And another thing – the tech shirts totally stink in about 5 minutes flat if you try to wear them as non-running just hang around t-shirts! So here is one positive thing: I often start runs with a tech shirt, get too hot and tear them off after about 10 minutes – a tech shirt tucked into the butt side of my shorts is a lot lighter than a sweat-soaked cotton/poly job.

  5. JeffM
    May 18, 2013

    If it’s warm, I prefer no shirt. Best to leave the body to regulate itself. I use tech fabrics in the gym to KEEP me warm. As a former Floridian, I can swear by the white cotton tee as coolest. I bought all my tech gear for cold, wet conditions. wicks sweat away so i stay warm.

  6. Juha Myllylä
    May 18, 2013

    Mike M: Evaporation of sweat is what causes cooling effect, not transfer of sweat from body.

    Moisture wicking shirt works by removing sweat (by capillary action) from skin to surface of shirt, where it will evaporate faster because of larger evaporative area. Evaporation of sweat then cools the shirt. With cotton shirt sweat remains between skin and shirt and evaporation is slower because level of humidity inside shirt is high. When enough sweat has gathered that shirt becomes soaked through, evaporation gets faster and because conductive properties of soaked shirt are greater than dry shirt, cooling effect is greater. That’s just me thinking about it, no science.

    I would suggest testing it. Like wrapping testing materials on thermometer and then pouring some water of same temperature and same amount on them, then just see what kind of difference there is. Or Perhaps it would better simulate perspiration if you would dip thermometer into water and then wrap it with test materials. And of course, use thermometer without any wrapping to see if it’s better to not wear shirt of any kind.

    • Jason
      May 18, 2013

      This is an excellent point. It’s important to note sweating cools the body because it evaporates on the skin, not because it mechanically removes heat from the body. A lot of people seem to be confused about that. Anything that interferes with evaporation is going to inhibit cooling, which includes high relative humidity or any fabric.

      • Juha Myllylä
        May 18, 2013

        Ah well, I would guess that conductive properties of technical shirt isn’t bad either, because sweat goes through small tubes in shirt ( & ) so that it technically has direct conductive tube of water to inside of shirt… Complicated it is. 😉

        And then there’s the thickness of fabric, cotton shirts tend to be much heavier and thicker than tech shirts. Tech shirts also might have vent holes. So it would probably be better to compare tech shirt to cotton mesh shirt. I would guess that mesh shirt would be better than tech shirt because it doesn’t transfer sweat away from body and let’s in air so that evaporation of sweat is more efficient than with normal cotton shirt.

        That was just for comparisation of different shirt materials, without shirt it is still better, because airflow is higher and sweat evaporating directly from skin directly cools the skin, no cooling is lost on surrounding air. And of course other means of cooling the body (like heat radiation) is more efficient without shirt.

        • Jason
          May 18, 2013

          Agreed about the thickness of the shirt. The wider the weave, the more air that can circulate, the more on-skin evaporation that will occur. And it would be less likely to reflect the radiated heat from the body.

          Regarding the shirt capillaries- it would be really fascinating to measure the heat conductivity of the sweat as it travels away from the body. This would be especially interesting if compared to a saturated hydrophillic material.

  7. David
    May 18, 2013

    Cotton absorbs sweat and starts to feel like an anchor, especially in hot weather. I don’t see how that can be good for you.If you’re exercising, your body temperature will increase. It won’t matter what you’re wearing. I don’t see how a soaking wet cotton tshirt would do anything but trap that heat. Maybe someone should do some real research on tech fabrics and what they really do, or don’t do, with regard to thermoregulation. Until then, I don’t see the point in becoming alarmist about tech fabrics.

    • Jason
      May 18, 2013

      Cotton is only better in that it becomes saturated faster, which promotes more efficient heat transfer (water soaked shirt transfers heat better than a dry shirt.) My recommendation would be to wear as little clothing as possible in hot conditions. A wide-brimmed hat would be far more effective than any shirt.

      • David
        May 18, 2013

        Thanks, I’ll try it. I’m slowly getting to the point where I can go topless without being too embarrassed.

  8. Mike M
    May 18, 2013

    Here’s a problem I have with this post and the ones you did last year: you note that he literature you looked at doesn’t go into thermoregulation at all. AT ALL. In other words, you haven’t provided any evidence that tech shirts are bad for thermoregulation just like the companies making them haven’t provided any evidence that they are good for it. If we shouldn’t believe them (which I agree with), why should we believe you(particularly when your thesis goes against common sense from a heat transfer perspective)?

    • Jason
      May 18, 2013

      See Juha’s comment above. My thesis is based on the principles of heat transfer. It doesn’t take an engineering degree to understand the basics of conduction, convection, and radiation. The lone experimental method that seems to have studied this was done by a company that developed another supposedly better fabric.

    • Paul Wallis
      May 23, 2013

      Your question deals with burden of proof. If shirt manufacturers claim their shirts better thermoregulate the skin better than cotton, other fabrics or nothing at all they should provide some evidence for such. It’s reasonable to conclude (if there is no data) that they don’t do what they say until we have the evidence.