The shirt you wear can say some things about you, but apparently not how well you run in the heat. Researchers at the Bergische Universitat in Wuppertal, Germany found that whether you wore cotton or polyester didn't make a difference to running performance or body temperature in 90-degree heat.
Researchers put eight fit runners on a treadmill in an 89-degree room for 30 minutes, upping the speed and hill grade until they were exhausted – and sweaty. Each runner did the test four times, three in various grades of polyester shirts and once in cotton. Throughout the tests, monitors on the runners showed no differences in skin, core, and body temperatures, heart rate, oxygen uptake, and blood lactate concentration. Even time to exhaustion, rating of perceived exertion, and how hot the runners said they felt didn't change, no matter the shirt. At such warm temperatures, "the body basically has no chance to dissipate heat," says Billy Sperlich, the study's lead author, who was surprised by the findings.
But don't chuck out all your technical clothing just yet. The polyester shirts did reduce one thing: relative humidity at the chest and back. During recovery, this promoted lower body temperature, less clothing wetness, and less shivering. Sperlich also thinks that at more moderate temperatures and over longer running times, the polyester shirts would show performance benefits.