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		3. Types of muscle fiber Humans have 3 types of muscle fibers that differ in their speed of shortening (fast vs slow) and major pathway of ATP production (glycolysis vsoxidative phosphorylation). The speeddepends on the type of myosininvolved, while the use of oxidative phosphorylationdepends on the presence of mitochondria and a store of oxygen. a. Fast oxidative (FO) i. Myosin isoform with high ATPase activity ii. Synthesize ATP using oxidative phosphorylation iii. Lots of mitochondria and myoglobin ("red" muscle) iv. Highly vascularized v. Intermediate diameter b. Fast glycolytic (FG) i. Myosin isoform with high ATPase activity ii. Synthesize ATP using glycolysis iii. little mitochondria and myoglobin ("white" muscle) iv. Less vascularized v. Larger diameter vi. Generate greater maximum tension, but fatigue easily c. Slow oxidative (SO) i. Myosin isoform with lower ATPase activity ii. Synthesize ATP using oxidative phosphorylation iii. Lots of mitochondria and myoglobin ("red" muscle) iv. Highly vascularized v. Small diameter vi. Sustain long contractions without fatigue Fatigueis the decrease in response of a muscle to prolongued stimulation, such as when your muscles "give out" carrying a heavy load. The muscles of humans contain different proportions of these fiber types. For example, SO fibers predominate in back muscles that hold your skeleton erect. The percentage of different fiber types is also different between individuals. "Typical" humans have an approximately equal distribution of FG and SO fibers in muscles like the thigh. Sprinters and weight lifters have a higher percentages of FG fibers in their thigh muscles. World class marathoners and cross-country skiers can have up to 95% SO fibers in their leg muscles.