Under Armour ColdGear Reactor Run Long Sleeve Top Men's Taking over as the next big technical running top, the Under Armour Reactor Run Long Sleeve Top for men features well-crafted technology. The ColdGear® Reactor technology offers Smart Insulation that provides adaptive, balanced warmth during any level of activity. This lightweight, fitted style is made from 4-way stretch fabric for ultimate freedom of movement. This cold weather top is perfect to wear on its on or as a layering piece.