Type II b fibers mainly use anaerobic glycolysis as their source of ATP. They have a large diameter, and high amounts of glycogen used in glycolysis to generate ATP for quick and high levels of tension. Since they do not use aerobic metabolism, they do not possess large numbers of mitochondria nor large amounts of myoglobin; thus, their white color. Type II b fibers are used to produce quick, forceful contractions to make rapid, powerful movements. They get tired quickly and their power lasts for short periods. Muscles generally possess a mixture of each type of fiber. The main fiber type present in a muscle is determined by the primary function of the muscle. Fast glycolytic fibers are thicker and have bigger potential for growth. Even though quick powerful movement training does not change the amount of type II b fibers, it changes their size. Their mass fraction thus changes in the muscle, allowing the muscle to grow.
? White in color due to very low myoglobin concentrations.
? Fatigue very easily.
? Contains low amounts of mitochondria.
? Have more sarcoplasmic reticulum.
? Fast rate of contraction for short periods.
? Produce high amount of power when contracted
? Used in short-term anaerobic activities like sprint and heavy weight lift,
(activities < 1 min).
? Also called Fast Twitch B fibers.
? Depend mainly on anaerobic oxidation (glycolysis) for energy production and accumulate lactic acid in considerable amounts during strenuous work.
? Higher myosin ATPase activity than Type I fibers.
? Lower capacity for ATP production via oxidative phosphorylation than ‘red’ fibers.
? Sparser capillary network.