The overweighted beast of burden, or the overweighted slave, can for certain instants shift the physical load, and find some slight respite even in enforcing additional pain upon such a set of muscles or such a limb. Not even that poor mockery of relief could the wretched man obtain, under the steady pressure of the infernal atmosphere into which he had entered.
What a cool comparison. I've gone on some backpacking trips and know just what Dickens is referring to. A few hours in, your muscles are aching but you can keep shifting the weight around throughout the day to give yourself some relief at the expense of another spot of your back or your legs. Headstone being unable to find any relief in this way with the weight of his actions really shows how much he is suffering for his actions.