Well, I suppose that you are well aware that neglecting air friction the work done on the bar will be $MgL$. This much energy is spent by the person on lifting the bar.
This question might be helpful.
It says the efficiency for a human body on average for cycling is 20%. So it is a safe bet to say that the man used $5MgL$ energy, spent $MgL$ on lifting the bar and $4MgL$ is wasted away.
Keep in mind that people spend energy just to live unlike machines, it is a much harder process to separate the amount of energy used for particular tasks from the total amount of energy used in a given amount of time.
A conservative force only returns the energy back when the object moves in a closed path, that is, it returns to the initial position (it doesn't matter if he returns due to other forces). This can be demonstrated as a theorem, but the intuitive explanation is that a conservative force depends only on the spatial coordinates, and not in the direction of motion (such as friction), and thus eventually when the body moves back the field force is in the opposite directions and makes work of the opposite sign, such that in a closed loop the overall work of the field over the particle is zero. The additional property for a field is that this happens regardless of the path taken (that is, you do not need to go back to the original position using the same path you used when moving forward. This can be shown to be the case when the force is described as the gradient of a potential.
Update:
I reread your question and I realized that I didn't actually answer your specific question (I misinterpreted the question).
Let us start with the easy case: a mass attached to a spring. Here your body is the mass and the agent is the spring. Describing the mass
moving in a conservative field instead of describing it in terms of mutual forces is much easier, as we do not have to take into account
the details of the internal forces within the spring. The mass losses energy and this energy is stored in the spring, not in the mass.
However we can give an alternative description ignoring the spring and saying that the mass accumulated potential energy. This is very convenient,
but only if your are sure that the spring will always be attached to the mass (so you can describe the force from the spring as conservative). But if you cut the
spring when the mass is at rest, the mass will "suddenly lose" its potential energy (the force is no longer conservative), the energy was actually
in the spring an will be dissipated as heat (assuming the spring will eventually stop moving, as in a real spring).
In the case of long distance forces that cannot be switched off, such as gravity, the description is a little more complex. If you have a very large
mass as the agent, you can approximate it as not moving due to the reaction force from the object, and describe the object as moving in a potential
field where it stores potential energy. A more accurate description would be that the agent actually moves and gains this as kinetic energy (when the object moves "up",
the earth will follow it and also move up too so it gains kinetic energy). But this motion is so small that you do not take it into account in your description. For all practical purposes the
object sees the agent as being at rest. Of course, I am assuming that you can move the object with a force that doesn't interact with the agent, otherwise it is the same but this time
the agent might be storing the energy in some different way, however the details do not matter for all practical purposes.
Best Answer
Assuming your question is about the concept of energy in physics:
The muscle actually uses chemical energy. How this works in detail is not a physics but a biology question. The chemical reaction will create heat and cause your muscle to contract. Consequently, your body loses chemical energy, that's why you have to eat, drink and breath, to keep these reactions going. In return you body loses heat energy to it's environment, as well as kinetic energy, which will be the actual movement of your arm.
If you do exercises, long term speaking, your body will not gain or lose any energy, it will become more efficient and capable of converting chemical energy to kinetic energy, potential energy, etc..
You might have a slight misunderstanding of what energy in a physics actually is. Wikipedia might help you.
http://en.wikipedia.org/wiki/Energy