Introduction Skeletal muscle consists of two types of muscle fibers type I type II type IIa type IIb Muscle fiber types vary across several characteristics resistance to fatigue color determined by an amount of myoglobin type of metabolism aerobic vs. anaerobic determined by relative amounts of mitochondria and glycogen Non-athletes will have approximately equal amounts of type I and type II muscle fibers Athletes will have different ratios of type I and type II muscle fibers depending on their predominant activities Type I Muscle Fibers Also known as slow twitch muscle fibers Color red more myoglobin Fatigue tolerance high able to maintain sustained contractions higher proportion in endurance runner's muscles Type of metabolism aerobic contains more mitochondria to carry out oxidative phosphorylation contains less glycogen Function postural control walking Mnemonic "One Slow Red Ox" = type I muscles are slow twitch, red, and use oxidative metabolism Type II Muscle Fibers Also known as fast twitch muscle fibers Color white less myoglobin Fatigue tolerance low best suited for short high-intensity contraction higher proportion in sprinter's muscles Type of metabolism anaerobic contains less mitochondria contains more glycogen to carry out glycolytic metabolism Subtypes type IIa "fast twitch oxidative" aerobic and anaerobic metabolism intermediate contraction strength type IIb "fast twitch glycolytic" anaerobic metabolism only highest contraction strength Function power activities sprinting powerlifting