On Food and Cooking

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 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.