Under Armour HeatGear® Mock Long Sleeve Men's. HeatGear® is our original performance baselayer. This shirt is made with super-light HeatGear® fabric that delivers superior coverage without weighing you down. It features mesh underarm & back panels for strategic ventilation, and the material wicks sweat & dries really fast. The ergonomic design keeps seams off high abrasion areas & increases durability. The hybrid raglan sleeve construction adds range of motion & comfort.