An air rowing machine coz it also helps to build muscle tone in the entire body as you are pushing resistance with both the leg muscles and the muscles of the entire upper body as well. You can build muscle while getting an aerobic workout, something you can't really do with a treadmill and which is a lot harder with a bike or elliptical machine. All of the stepping or walking type of exercise machines are limited to your weight. Unless you are willing to strap on twenty or thirty pounds of weights, building leg muscle mass past a certain point is just not possible.

read that wile looking for the best way to excerzize the whole body wivout over doing it