As far as I know the glucose gets processed from carbs and fats and as a result carbon dioxide, water and "energy" are released. The carbon dioxide and water are expelled by the body, and then energy is used by the muscles. But how exactly is it used? What happens to this "energy" and what is it exactly?
You can break down the energy expenditure of the human body in terms of the different organs which use it up: this is done in this biochemistry book for instance, http://www.ncbi.nlm.nih.gov/books/NBK22436/
You can also look at the detailed expenditure of each cell: although the amounts will vary according to cell type (neuron vs. muscle cell, say!), the "budget items" are the same. See this thread: Breakdown of energy expenditure at the level of a single cell