In the 1950s, primitive gym treadmills were not motorized. They depended upon leg power to move the belt. Guess what? Non-motorized treadmills are back and they are popular in high-end gyms and CrossFit competitions. Today's non-motorized treadmills are more high-tech than the originals. People move the belts by moving their feet, but the belt speeds up as the person reaches the top of the treadmill, and slows down when he or she drifts backward. The level of difficulty is about 30 percent greater with non-motorized treadmills at the same relative speed. The belt speeds up or slows down according to the effort of the exerciser, which makes them excellent for high-intensity interval training workouts. They also cut down on injuries: the belt stops when the person stops.