What does the bent body of the man with the hoe signify?

The man has been bent and bowed by the figurative weight of responsibility on his shoulders. A life of work is all he knows, and he bears the "burden of the world." His life, nothing but toil, has "made him dead to rapture and despair" so that he has become a "thing that grieves not and that never hopes." Toil is his entire life, to the point that he does not even feel—what would be the point of feeling? It does not lessen one's burden or lighten one's work. He is almost an animal, "a brother to the ox," because he has been stripped of his humanity. We are meant to do so much more than work. The "light within this brain" has been blown out, and he seems, no longer, to resemble the "Thing the Lord God made and gave / To have dominion over sea and land." He is no master now. He feels no "passion," and he asks no questions of the "heavens." Out of all the things a person is capable of doing, he only does one: work. He is a "Slave of the wheel of labor" and no more. He does not think or wonder or dream. His "aching stoop" signifies the tragedy of his lowly condition when he might have been, and should have been, so much more.

In Markham's "The Man with the Hoe," the man is bent over because of "the weight of the centuries" (line 1) that he bears and because of the weight of the "the burden of the world" (line 4), which presses down upon him. This is a homage and a lament for the working man throughout time, who toiled in the fields or a steel mill, oppressed by the wealthy few, who have through their greed oppressed working people, taken away the light and music in their souls, condemning them to long, grueling days and nights of work, for virtually nothing in return. This poem is based upon a painting of the same name by Jean-Francois Millet. The painting portrays a man who is bent over, working a rocky ground. The plight of the working man has not really changed much throughout history. In spite of modern features such as minimum wage and OSHA regulations, many working people are still bent over metaphorically, if not literally.
http://www.getty.edu/art/collection/objects/760/jean-francois-millet-man-with-a-hoe-french-1860-1862/