Skeletal muscle fibers can be classified based on two criteria: 1) how fast do fibers contract relative to others, and 2) how do fibers regenerate ATP.  Using these criteria, there are three main types of skeletal muscle fibers recognized (Table 10.5.1). Slow oxidative (also called slow twitch or Type I) fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative (also called fast twitch or Type IIa) fibers have relatively fast contractions and primarily use aerobic respiration to generate ATP. Lastly, fast glycolytic (also called fast twitch or Type IIx) fibers have relatively fast contractions and primarily use anaerobic glycolysis. Most skeletal muscles in a human body contain all three types, although in varying proportions.

The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibers hydrolyze ATP approximately twice as rapidly 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 primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways, then it is classified as oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.

Slow oxidative fibers have structural elements that maximize their ability to generate ATP through aerobic metabolism.  These fibers contain many more mitochondria than the glycolytic fibers, as aerobic metabolism, which uses oxygen (O₂) in the metabolic pathway, occurs in the mitochondria. This allows slow oxidative fibers to contract for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and thus do not produce a large amount of tension.

In addition to increased numbers of mitochondria, slow oxidative fibers are extensively supplied with blood capillaries to supply O₂ from the bloodstream.  They also possess myoglobin, an O₂-binding molecule similar to hemoglobin in the red blood cells. The myoglobin stores some of the needed O₂ within the fibers themselves and is partially responsible for giving oxidative fibers a dark red color.

The ability of slow oxidative fibers to function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints.  Because they do not produce high tension, they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.

Fast glycolytic fibers primarily use anaerobic glycolysis as their ATP source.  They have a large diameter and possess large volumes of glycogen which is used in glycolysis to generate ATP quickly.  Because of their reliance on anaerobic metabolism, these fibers do not possess substantial numbers of mitochondria, a limited capillary supply, or significant amounts of myoglobin, resulting in a white coloration for muscles containing large numbers of these fibers.

Fast glycolytic fibers fatigue quickly, permitting them to only be used for short periods.  However, during these short periods, the fibers are able to produce rapid, forceful contractions associated with quick, powerful movements.

These different fiber types can be easily identified in poultry. Imagine a turkey. The legs and thighs of the turkey are dark meat, due to their slow oxidative fibers and robust supply of blood vessels and myoglobin. Turkeys spend most of their days walking around looking for food, so their legs must be able to work all day without fatiguing. Alternately, turkey breast is white meat, due to its fast glycolytic fibers and relatively insubstantial supply of myoglobin and lesser blood supply. Turkeys do not fly long distances, but only need to get into trees to roost. Their breast tissue produces strong, rapid contractions, but only for very brief flights.

Fast oxidative fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between slow oxidative fibers and fast glycolytic fibers. These fibers produce ATP relatively quickly, and thus can produce relatively high amounts of tension, but because they are oxidative, they do not fatigue quickly.  Fast oxidative fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement.

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