![]() ![]() The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time. The myoglobin stores some of the needed O 2 within the fibers themselves (and gives SO fibers their red color). The SO fibers also possess myoglobin, an O 2-carrying molecule similar to O 2-carrying hemoglobin in the red blood cells. SO fibers are extensively supplied with blood capillaries to supply O 2 from the red blood cells in the bloodstream. The SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. The oxidative fibers contain many more mitochondria than the glycolytic fibers, because aerobic metabolism, which uses oxygen (O 2) in the metabolic pathway, occurs in the mitochondria. As a result, glycolytic fibers fatigue at a quicker rate. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. ![]() If a fiber primarily produces ATP through aerobic pathways it is oxidative. The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. Fast fibers hydrolyze ATP approximately twice as quickly as slow fibers, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the center of the sarcomeres at a faster rate). The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Most skeletal muscles in a human contain(s) all three types, although in varying proportions. The FG fibers fatigue more quickly than the others. Lastly, fast glycolytic (FG) fibers have fast contractions and primarily use anaerobic glycolysis. ![]() Fast oxidative (FO) fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers. Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Using these criteria, there are three main types of skeletal muscle fibers. Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP.
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