The muscles of humans and other mammals are only about 20% efficient at turning food into mechanical work. Overall efficiency is even worse when you consider re

The muscles of humans and other mammals are only about 20% efficient at turning food into mechanical work. Overall efficiency is even worse when you consider resting metabolism during non-work times, such as when people are asleep. That profligate waste is happening on top of the inefficiency of using plants to produce human-edible "fuel" for workers; less than 5% of incoming solar energy gets stored as something humans can eat, or a bit better if you include parts that ruminants can eat. Motors running from battery-backed solar power can do much better than that. Cheap solar modules made of multicrystalline silicon can convert 16% of incoming solar energy to electricity, and after going through a battery and a motor you can still get 80% of the original electricity back as mechanical work. A solar farm "feeding" electrical machinery is 1-2 orders of magnitude more productive on an areal basis than a plot of land feeding manual laborers with human foods. Additionally, the solar farm doesn't need weeding, irrigation, fertilizer, or fertile soil. A reversion to muscle power would be expected following a massive societal collapse, but it would not be a rational plan for a high-tech society to increase its efficiency and sustainability.