The resistance of your rowing machine impacts how it moves. Select from hydraulic and magnetic rowing resistance. Hydraulic rowers are typically more compact in size, while magnetic machines make little noise as they operate. Details like easy-to-reach tension knobs or buttons allow you to increase or decrease the intensity of your resistance as you train. From a fitness perspective, the WaterRower Natural works 84 percent of your muscle mass, helping tone and strengthen your muscles while burning far more calories than most other aerobic machines. The exercise is also low impact, as it removes all the body weight from the ankles, knees, and hips, but still moves the limbs and joints through a full range of motion--from completely extended to completely contracted.