Why does holding an object cost energy while no work is being done? I was reading the discussion here:
Why does holding something up cost energy while no work is being done?
I feel as though the question is being avoided. 
Suppose instead of holding an object by hand we would attach some machine which burns fuel and converts it to force. This force is equal to mg.
We would like to predict how much fuel is being burnt (how much energy is spent) in order to hold the object for a time of T seconds.
The attached link does not give any clue how one can approach this problem.
Furthermore, and this is what bothers me most, suppose that while the machine holds the object perfectly still someone would miraculously turn off the earth's gravity. The machine still creates a force of mg. Suppose that after time of T seconds the object is at height h (relative to its starting position). Would we then be able to say the amount of energy spent by the machine is mgh? Maybe one would say it depends on how the machine works and how much energy is converted to heat. So suppose the machine has perfect efficiency (equals or very close to 1), i.e. every joule spent by the machine is dedicated to hold the object.
For me it seems the 2 scenarios are exactly alike, since the machine is indifferent to gravity.
Please be the devil's advocate when answering this question :)
 A: To understand why holding objects costs energy even though the work appears to be zero, you have to understand how muscles work.
When you are holding an object, your muscles are contracted. The process of muscle contraction consists in a protein filament called Myosin pulling another filament, called Actin. Since this is a dynamical process (the Actin filaments are moving because the Myosin filaments exert a force on them), you can see how the work is not zero.
In this video you can get a fairly good idea of the process: https://www.youtube.com/watch?v=Cjx3vSm54N8
As for your thought experiment, you should be clearer on how the machine works. Of course you don't really need a machine, since a piece of rope will basically serve the purpose perfectly. But let's say you want to do it with a machine, because you have a lot of fuel to spare.
We can imagine, for example, an "artificial muscle" that basically works like ours. In this case, when gravity is switched off, if the muscle stays contracted, work will still be done, even if nothing will take off or start to move; but of course, you can just relax the muscle and still be able to hold the now weightless object. 
But we can imagine different kinds of machines: maybe a levitating machine using jet propulsion to counterbalance the force exerted by the object. In this case, your reasoning will be basically correct: when gravity is switched off, the machine will take off, reaching a certain height in a certain amount of time and giving us an idea of the amount of work necessary to hold the object using this particular machine (but in this case, remember to take also kinetic energy into account!). 
