White Muscle Fibers White muscle fibers specialize in exerting force rapidly and briefly. They are fueled by a small store of a carbohydrate called glycogen, which is already in the fibers, and is rapidly converted into energy by enzymes right in the cell fluids. White cells use oxygen to burn glycogen, but if necessary they can generate their energy faster than the blood can deliver oxygen. When they do so, a waste product, lactic acid, accumulates until more