Question related to work done and energy Suppose you lift a box from the ground over your head. You will be using energy as there is work done. But when you keep the box lifted over your head, aren't you using any extra energy?
 A: Yes, your body is utilizing stored chemical potential energy when you hold a box above your head.
Our muscles work in a funny way: it takes energy for them to flex. In other words, it takes energy for our muscles to exert a force, even if there isn't any large-scale macroscopic movement, such as in your box example. For us muscled beings, there is actually movement happening on the cellular and even molecular levels. Note that, in the usual cases you study in classical mechanics (think of a spring), most forces can be exerted indefinitely since no energy is "being spent."
(This whole muscle contraction business is quite fascinating and is worth researching, but is better elaborated on by biology and chemistry. See here for a useful Google search.)
A: In short, No !
While you are lifting an objectupwards, you are moving it against the action of gravitational pull of the earth just like stretching a spring. While you are pushing it up you do work against gravity on the object which is stored in it as it's gravitational potential energy.
However when you have lifted the object to desired height and just keep it there, no work is done as no movement against any force is done. You may feel that you are doing work because of pain in your hand but pain is not characterised by work.
