The designs pictured above are not really capable of doing really heavy lifting. But, the concept is sound. Just think of what kind of power built the pyramids and ancient cities. That's right: good old human labor (maybe not with slave labor?). That underscores what we are capable of with our limited bodies, even before modern science steps in. If you wanted to, you could gear create a design geared down to the point that a human powered forklift could compete with electric and fossil-fueled versions... but it would be a slow process.