Let's say an object is held at rest by a force opposite of gravity for a given amount of time. Since it takes energy to hold the object then how can one calculate the energy needed to hold the object for x amount of time? Assume near 100% efficiency, or most of the force is used to hold the book and not create heat or anything else. The reason why I ask this is because I notice how tired I get holding things for a long time. Obviously I'm expending energy. Just wanted to know how much if my muscles were theoretically very efficient.