This document discusses work, power, energy, and efficiency in humans. It provides an example problem calculating the grams of fat an idle person would gain in a day consuming 2500 kcal of food (80g). It defines human efficiency as useful work output divided by energy input. Efficiency varies with activity, as it depends on how many muscles are used cooperatively to produce work compared to the energy used by other organs - the more muscles producing useful work, the more efficient the body is.