Arcteryx says about the Rho LT Bottom Men's
The Rho LT is your lightweight thermal base layer for low and medium output when it’s cold out. The odour-resistant Torrent™ fabric has a soft, brushed surface that speeds moisture transfer and delivers serious comfort next to the skin. Elastane fibres add stretch and work with the articulated patterning and gusseted crotch for freedom of movement. The waistband is designed for comfort and easy layering, and a smart thigh pocket stashes a smartphone. Updated with a resized side pocket that accommodates a smartphone, more comfortable trim fit.