 Type I fibers or slow oxidative fibers are small fibers that appear red in fresh specimens and contain many mitochondria and large amounts of myoglobin and cytochrome complexes. Their high levels of mitochondrial oxidative enzymes are demonstrated by their strong succinic dehydrogenase and NADH-TR histochemical staining reactions as described previously (see Fig. 11.3). Type I fibers are slow-twitch, fatigue-resistant motor units (a twitch is a single, brief contraction of the muscle). These fibers have great resistance to fatigue but generate less tension than other fibers. Their myosin ATPase reaction velocity is the slowest of all of the fiber types. Type I fibers are typically found in the limb muscles of mammals and in the breast muscle of migrating birds. More importantly, they are the principal fibers of the long muscles of the back in humans, where they are particularly adapted to the long, slow contraction needed to maintain erect posture. A high percentage of these fibers make up the muscles of high-endurance athletes such as marathon runners.
 Type IIa fibers or fast oxidative glycolytic fibers are the intermediate fibers seen in fresh tissue. They are of medium size with many mitochondria and a high myoglobin content. In contrast to type I fibers, type IIa fibers contain large amounts of glycogen and are capable of anaerobic glycolysis. They make up fast-twitch, fatigue-resistant motor units that generate high peak muscle tension. Athletes who have a high percentage of these fast oxidative glycolytic fibers include 400and 800-m sprinters, middledistance swimmers, and hockey players.
 Type IIb fibers or fast glycolytic fibers are large fibers that appear light pink in fresh specimens and contain less myoglobin and fewer mitochondria than type I and type IIa fibers. They have a low level of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen. These fibers are fast-twitch, fatigue-prone motor units and generate high peak muscle tension. Their myosin ATPase velocity is the fastest of all the fiber types. They also fatigue rapidly as a result of production of lactic acid. Thus, type IIb fibers are adapted for rapid contraction and precise, fine movements. They constitute most fibers of the extraocular muscles and the muscles that control the movements of the digits. These muscles have a greater number of neuromuscular junctions than do type I fibers, thus allowing more precise neuronal control of movements in these muscles. Short-distance sprinters, weight lifters, and other field athletes have a high percentage of type IIb fibers.
The structural and functional subunit of the muscle fiber is the myofibril.
A muscle fiber is filled with longitudinally arrayed structural subunits called myofibrils (Fig. 11.4). Myofibrils are visible in favorable histologic preparations and are best seen in cross-sections of muscle fibers. In these sections they give the fiber a stippled appearance. Myofibrils extend the entire length of the muscle cell.
Myofibrils are composed of bundles of myofilaments.
Myofilaments are the individual filamentous polymers of myosin II (thick filaments) and actin and its associated proteins (thin filaments). Myofilaments are the actual contractile elements of striated muscle. The bundles of myofilaments that make up the myofibril are surrounded by a welldeveloped smooth-surfaced endoplasmic reticulum (sER), also called the sarcoplasmic reticulum. This reticulum forms a highly organized tubular network around the contractile elements in all striated muscle cells. Mitochondria and glycogen deposits are located between the myofibrils in association with the sER.
Cross-striations are the principal histologic feature of striated muscle.