By Emily Chen-Newton Zach Williams, a U.S. para alpine skier, lowers himself into his molding rig. Williams was already a licensed prosthetist when he was introduced to his sport: skiing in a seated position. A double amputee, he's walked on prosthetics since age two. Now his business helps other Paralympians get the right fit for their competitive gear. Emily Chen-Newton hide caption PARK CITY, Utah — At the Winter Paralympics in Milan and Cortina, some athletes use prosthetics. But, because every person's body is different, competitors sometimes need to modify or design their own specialized equipment to fit their own disability. And some athletes are entering the business of prosthetics. Zach Williams is a U.S. para alpine skier who missed these Games due to injury. He was already a licensed prosthetist when he was introduced to his sport, skiing in a seated position. After a double amputation, he's walked on prosthetics since age two.…