Cotton T-shirts from a soccer league may be fine to throw on after a workout. However, despite cotton's easy availability, it's not the best fabric for working out. Certain areas of your body benefit from 100-percent cotton, but the rest of your body may need a more advanced fabric.
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Cotton is breathable, which means it won't make you sweat more. It's also somewhat absorbent, so can soak up some of the sweat you generate during a workout. It's relatively easy to maintain, as cotton clothes can typically get thrown in the regular wash cycle with the rest of your wardrobe. Cotton workout clothes are also relatively inexpensive compared to synthetic fabrics.
On the downside, 100-percent cotton loses its shape, so your knees bag out even when standing in cotton workout pants. And, while it can absorb sweat, it isn't built to wick the sweat away, leaving you with a sticky sensation. It may be relatively easy to maintain, but cotton can shrink if washed or dried on too high a setting. While cotton clothes aren't expensive, they don't necessarily last as long as more advanced fabrics built specifically for working out.
Cotton in combination with another material such as Spandex alleviates any potential misshape, and Spandex prevents wrinkling. Nylon and polyester are also viable options -- they help circulate the air more effectively than cotton. However, pants should include a 100-percent-cotton gusset to minimize any potential for jock itch or yeast infections.
When buying workout gear, you need clothes that fit relatively close to the body. Close-fitting clothing is also safer than baggy clothes, as it won't get caught in equipment such as elliptical poles or biceps curl machines. It's also important to wear close-fitting workout clothes if you're taking a class, because your instructor may want to view your form to ensure you're completing moves correctly and using the correct techniques.