By Harold McGee
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 oxygen arrives. This accumulation of lactic acid limits the cells’ endurance, as does their limited fuel supply. This is why white cells work best in short intermittent bursts with long rest periods in between, during which the lactic acid can be removed and glycogen replaced.