The term cardio emerged as a replacement for the term aerobic; cardio exercise basically means any exercise that benefits the heart. Therefore, cardio fitness equipment is any variation of exercise equipment that ramps up the body's consumption of oxygen, which in turn exercises the heart. In order to do so, the cardio fitness equipment must create a sustained activity that raises the heart rate and encourages the body to take in increased amounts of oxygen.
An example of a piece of cardio fitness equipment is a stationary bicycle. The stationary bike allows the rider to participate in a sustained moderate effort, thereby raising the heart rate enough that oxygen is the primary source of the body's energy. The prolonged, constant exercise is unlike, say, weight lifting, which is a short, intense activity that uses energy in short bursts. Such activities will generally use stored glycogen as its energy source rather than oxygen.
Another example of cardio fitness equipment is the treadmill. The treadmill is a flat platform with a moving belt that allows the user to run in place. This prolonged aerobic activity allows the heart rate to be raised to an optimal level. Treadmills often allow the user to adjust the speed at which the belt moves, as well as the incline of the belt. This makes running uphill or downhill possible.
Treadmills and elliptical machines are somewhat similar machines, but ellipticals have individual platforms, one for each foot. One end of each platform is connected to a wheel and the other end is connected to a fixed point. This creates an elliptical motion when the user propels the platforms forward, mimicking a running or climbing motion. The elliptical machine is designed to exercise a broader range of muscles than the treadmill, and the is a lower-impact piece of cardio fitness equipment — that is, the harsh impact on the body is lessened by the smoother motion. Users with bad knees or other joints might be better off using the elliptical machine rather than the treadmill.
A piece of cardio fitness equipment that works the upper body is called a rowing machine. This piece of equipment mimics the motion of rowing a boat. The user sits in a saddle attached to a beam. The saddle is allowed to slide back and forth as the user "rows" using fixed or movable arms with hand grips. This action allows the user to work both leg muscles and arm muscles, as well as chest and back muscles.