I can tell from experience that it is really easy to bake normal maps. (the technical part and some minor changes like veins and better body definition around critical areas like knee or ankle; never tried to do a full muscle body normal map)
You need only one low-poly mesh (the mesh, you will apply the normal map to) and a high-poly one for resculpting (add/subtract: crease brush, f sculpt draw and blob etc.)
The high poly mesh can be generated by applying one or two subdivisions on a low-poly mesh.
At first create a 1024x1024 or a 2048x2048 blank texture for the low-poly mesh (select whole low-poly mesh in edit mode and go to the uv/image editor-->hit the "new" button, give the image a name and confirm).
After that go to object mode, shift select the high-poly mesh and the low-poly mesh, go to the render tab (right side: camera symbol), open the bake scroll down menu, change bake mode to normals, check "select to active"-box and
set the margin to 10 pixels (depending on the uv layout), click bake and voila.
Here a link to my thread about this topic with a link to a good tutorial:
viewtopic.php?f=22&t=17976