Expedition Weight Long Underwear Bottom Pant, Polypropylene & Lycra Baselayering 95% Polypropylene/5% Lycra featuring 5 fabric treatments HeelClench System features a Lycra stirrup that fits on the heel to prevent bunching when adding outerlayers. Reinforced Knees for added durability and impact protection. Front Flap features a double layer of fabric to protect from drafts. Tagless Label & Flat Seams for added comfort. Diamond-gusset Crotch enhances movement.