Under Armour HeatGear Sonic Armourvent Short Sleeve Running T-Shirt Body: 90% Polyester/10% Elastane, Back Insert: 100% Polyester Smooth Fabric - An ultra-soft and smmooth fabric that provides extreme comfort with very little weight. 4-Way Stretch Fabrication - Allows greater mobility and maintains shape. ArmourVent - Delivers true breathability in a light, stretchy, durable, fast-drying fabric. Signature Moisture Transport System - Wicks sweat away from the body, keeping you cooler and drier. Smooth Flat Lock Seams - Allow a full range of motion without chaffing.