Everyone knows that walking uphill is much more tiring than walking on level ground, like a flat road or sidewalk. So what is it that makes walking uphill so much harder?
It has to do with gravity. Earth’s gravitational field is always pulling us toward the center of the Earth, and our feet have to exert an equal and opposite force to keep us upright.
That’s happening when we’re just standing upright or walking horizontally across a level surface, like walking down a flat street. But when we walk up an incline—along with all the effort we’re exerting to walk forward—we’re also lifting our bodies against gravity.
That’s also the reason why walking up stairs is so much harder than walking down stairs—when we go up, we have to work to overcome the force of gravity, while when we go down, the pull of gravity makes it easier for us. This means that all of the work we’re doing when we’re climbing uphill makes us use more energy, which is why we get tired so fast.