UA Base™ 4.0 is built for extreme cold weather and lower activity. Their unique pattern traps heat without adding bulk. Think long underwear, revamped for today's top athletes.. UA Base™ 4.0 Extreme Baselayer is our warmest yet, delivering extreme warmth in the extreme cold. Soft, brushed grid interior traps air, managing your body's microclimate to keep you warm & comfortable. Material wicks sweat & dries really fast. 4-way stretch material moves better in every direction. Dropped, shaped hem. Raglan sleeve. 81% Polyester/19% Elastarell.