A “white” muscle fiber characterized by its fast speed of contraction and a high capacity for anaerobic glycolysis.
Muscle fibers that have a high contractile speed are considered fast-twitch and are also known as type II fibers. Type II differ from type I fibers, which are considered slow-twitch. Type II fibers are typically classified based on their preferred manner of energy utilization. For example, type Ha fibers, also known as fast- twitch red muscle fibers, produce energy via both the aerobic and anaerobic metabolism of glucose. These fibers differ from type lib fibers, also known as fast-twitch white fibers, which produce energy primarily via aerobic metabolism. Due to their tendency to produce energy anaerobically, these fibers are often called fast-glycolytic (FG) fibers, while type Ila fibers are called fast-oxidative glycolytic (FOG) fibers.