According to the Sports Goods Manufacturers Association, treadmills are the #1 home-fitness item, outdistancing stair-climbers, stationery bicycles, rowing machines and cross-country skiers. In fact, some surveys indicate that nearly 40 million Americans run on treadmills.Marathoners use treadmills, so do collegiate runners, soccer moms, lunchtime joggers and legions of senior citizens who are eager to get in shape and keep in shape. And according to the research firm American Sports Data, women.