FUNCTIONAL TYPES OF SKELETAL MUSCLE FIBERS

Slow oxidative fibers/slow twitch fibers are

red muscle fibers commonly found in the muscles of the back. They contain large amounts of myoglobin and many large mitochondria and use aerobic cellular respiration to generate ATP. Slow oxidative fibers are the smallest in diameter and are relatively weak, so they take longer than fast oxidative-glycolytic and fast glycolytic fibers to reach tension. They are very resistant to fatigue, and as a result, are capable of providing sustained muscle contractions.

Fast glycolytic fibers/fast twitch B fibers are

white muscle fibers with a low myoglobin content. This type of muscle fiber contains large amounts of glycogen and as such mainly generates ATP by glycolysis. Fast glycolytic fibers contract forcefully and quickly generating the most powerful contraction. These fibers are the largest in diameter, and contain the most myofibrils. Unlike the other muscle fiber types, fast glycolytic fibers fatigue very quickly.

Fast oxidative-glycolytic fibers/fast twitch A fibers are

red muscle fibers that contain a high myoglobin and blood vessel count. They generate ATP by aerobic cellular respiration, and anaerobic glycolysis due to their high intracellular glycogen levels. Fast oxidative-glycolytic fibers are intermediate in diameter, and are faster than slow oxidative fibers, and slower than fast glycolytic fibers to reach tension. They are fairly resistant to fatigue, and as a result are not able to maintain this tension for as long as slow oxidative fibers.

three primary types of muscle fiber that make up skeletal muscle tissue:

slow oxidative fibers, fast oxidative-glycolytic fibers, and fast glycolytic fibers

muscles which are constantly used, such as the muscles of the back involved in maintaining upright posture, have a high proportion of slow oxidative fibers, whereas muscles that are only less regularly used, such as

those needed to throw a ball, have a higher proportion of fast glycolytic fibers.