I've devised a world where artificial intelligence (AI) have taken over as middle-class (America), leading to the further division of human classes: a superior upper-class and a now much lower-class (slaves).
In this world the natural resources normally available to manufacture electricity for consumption are (nearly) wiped out, and our AI middle-class are requiring their lower-class humans to perform some sort of manual labor for production of electricity. This output of electricity is imperative for the AI and upper-class to sustain their own lives.
There are many methods available to create electricity by manual labor, but my question is which method is considered the most efficient with special consideration given to the human body's physical ability.
The AI want to make the best use of their resources (i.e. food for the lower-class, maximum output/human), thus needing a very efficient method for the production, while making the assumption the lower-class humans performing this labor are fed special nutrient rich diets to maximize output. These humans are born and bred to perform these jobs, and are likely to not know any other style of life.