The wheels may be spinning, but business for stationary fitness equipment isn't gaining much ground. Participation in and sales for treadmills, cross-country ski machines, and stationary cycles are as flat as the floor they sit on. According to the Sporting Goods Manufacturers Association, expenditures on treadmills held steady, with consumers spending about $1.6 billion in both 1997 and 1998. Stationary cycles had the next best showing, declining just 3 percent in the same time period, to $380 million. Spending on cross-country ski machines dropped 7 percent from 1997 to 1998, to $520 million.