the action of your feet against the deck moves the belt. The belt only moves if you move it. Both runners and walkers expend more effort on a curved belt non-motorized treadmill than on a motorized treadmill. This has fitness benefits as users can get their heart rates into a higher zone at a lower speed. One study found that subjects expended 30 percent more energy at the same speed. But the trade-off was that they preferred to run or walk at a slower speed so as to keep their exertion constant.