A treadmill is a piece of fitness equipment that simulates running and walking while staying in one place by providing a moving platform, a conveyor belt, that turns in place over a flywheel; the flywheel is driven by an electric motor. The word treadmill traditionally refers to a type of mill which was operated by a person or animal treading steps of a wheel to grind grain. The belt moves to the rear allowing a person to walk or run an equal, and necessarily opposite, velocity. The rate at which the belt moves is the rate of walking or running. Thus, the speed of running may be controlled and measured. The more expensive, heavy-duty versions are motor-driven. The simpler, lighter, and less expensive versions passively resist the motion, moving only when the walker pushes the belt with their feet. As a cardiovascular exercise treadmills offer the benefit of reduced impact with built in shock absorption systems. Exercising on a treadmill can reduce the strain to the ankles, knees and lower back that would be involved in running on a normal surface. As a cardiovascular exercise running on treadmills is easier than running equidistant outdoors because the running platform is smooth and there is no wind resistance. A person running outdoors will typically expend up to 10% more energy than a treadmill runner.