Fatty acids from fat reserves in muscle and other parts of the body are also used by Aerobic respiration to produce ATP. Adenosine triphosphate, or ATP is the way your body uses biochemicals to store and use energy. The intensity or how hard you exerted yourself while performing that intense bout of activity has everything to do with whether or not sufficient oxygen was able to be transported to the mitochondria of all your cells to produce that all-important ATP needed in order for your muscles to contract.