Under Armour Knockout Tank Top. This is one of the lightest, most breathable shirts we make—and your new go-to. Any sport, any training, any time. It features incredibly soft, breathable fabric, 4-way stretch construction that moves better in every direction, and material that wicks sweat and dries really fast. The design includes T-back straps with wordmark taping details and a dropped, shaped hem for enhanced coverage.