Fast-twitch fibers

Figure22. (a) Euthyroid(normal)ratmuscleshowingmixtureoftype1 (slow-twitch) and type 2 (fast-twitch) fibers, (b) Hypothyroid rat muscle showing uniformly type 1 (slow-twitch) histochemical profile myofibrillar ATPase after alkaline preincubation.

Skeletal muscle contains different types of fibers primarily suited to anaerobic (fast twitch fibers) or aerobic (slow twitch fibers) conditions. 

There is probably no biological phenomenon that has excited more interest among biochemists than the movement caused by the contractile fibers of muscles. Unlike the motion of bacterial flagella, the movement of muscle is directly dependent on the hydrolysis of ATP as its source of energy. Several types of muscle exist within our bodies. Striated (striped) skeletal muscles act under voluntary control. Closely related are the involuntary striated heart muscles, while smooth involuntary muscles constitute a third type. Further distinctions are made between fast-twitch and slow-twitch fibers. Fast-twitch fibers have short isometric contraction times, high maximal velocities for shortening, and high rates of ATP hydrolysis. 

Mammalian skeletal muscle can be separated Into two distinct fiber populations, based on relative contraction characteristics, and are referred to as slow-twltch (Type I) or fast-twitch (Type II) fibers. The slow-twltch fiber type exhibits a relatively low shortening velocity (27), a low rate of tension development (27). a low myosin ATPase activity (28) and a low rate of calcium sequestration by the sarcoplasmic reticulum (29). The converse Is true for the fast-twitch fibers. Since contraction velocity highly correlates with myosin ATPase activity (30), It Is possible to easily Identify,... 

Schematic representation of the internal organization of a muscle fiber. Note the parallel arrangement of myofibrils and the location of the sarcoplasmic reticulum and T-tubules with respect to them. This arrangement is typical of a mammalian fast-twitch fiber, with the T-tubules and terminal cisternae located at or near the Z-lines.

Muscle fibers can be classified as either fast-twitch or slow-twitch. The slow-twitch fibers, or type 1 fibers (also called slow-oxidative), contain large amounts of mitochondria and myoglobin (giving them a red color), utilize respiration and oxidative phosphorylation for energy, and are relatively resistant to fatigue. Compared with fast-twitch fibers, their glycogen content is low. The slow-twitch fibers develop force slowly but maintain contractions longer than fast-twitch muscle. 

Would you guess the efficiency of fast twitch fibers to be greater or less than that of slow twitch fibers ... 

Importantly, the precise electrical stimulus protocol chosen also impacts the training effect on the muscle. In particular, the frequency of stimulus delivered likely determines the types of muscle fibers activated. Slow-twitch fibers normally have a firing frequency of -10 Hz, whereas fast-twitch fibers have a firing frequency of 30 Hz (97). A TCEMS stimulus frequency up to 10 Hz would, therefore, preferentially activate slow-twitch fibers and may selectively improve resistance to fatigue (101), whereas frequency greater than 30 Hz may activate both types of fibers or may selectively recruit fast-twitch fibers and... 

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