In the 1950s, first gym treadmills weren’t motorized. They were dependent upon leg power to move the belt. Guess what? A non-motorized treadmill became popular again in high-end gyms and CrossFit competitions.
Non-motorized treadmills today are more high-tech than the originals. Runners move the belts by moving their feet, but the belt speeds up when the person reaches the top of the treadmill and slows down when he or she drifts backwards.
The level of difficulty is about 30% greater with non-motorized treadmills at the same relative speed. The belt speeds up or slows down according to the effort of the runner, which makes them perfect for high-intensity interval training workouts. They also reduce the risk of injuries: the belt stops when the person stops.