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Two principal types of muscle cells or fibers that are randomly mixed in all human muscles can be differentiated on the basis of their colors, the quantities of mitochondria that they contain, and the speeds with which they contract.

Using two of these three characteristics, Dubowitz and Pearse (1960) proposed a classification in which Type I fibers were colored red and were found to have high concentrations of mitochondria. The redness was due to a high content of the protein myoglobin, which transfers the oxygen carried in blood to the mitochondria and also acts as an oxygen store in the muscles. The Type II fiber was white, due to a low myoglobin content, and had a low mitochondrial content. An important weakness of this classification is that it takes no account of the different speeds at which the different muscle fibers can contract or, in more scientific terms, the rate at which the different fibers can complete the “crossbridge cycle” of muscle contraction depicted in Exercises 1.3.

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Yet as early as 1873, the German physiologist Ranvier observed that “red” muscles contract and relax more slowly than “white” muscles. More recent work has confirmed this, so that the most modem classification of muscle fiber type is that fibers are either red, Type I, slow twitch (ST) fibers, or they are white, Type n, fast twitch (FT) fibers.

More recent evidence indicates that the speed of cross-bridge cycling in the different muscle fiber types is determined by the “strength” of the ATP-specific enzyme myosin adenosine triphosphatase (myosin ATPase), which sits in the myosin head and to which the ATP binds. Researchers have found that fast-contracting (FT) white muscle fibers have greater myosin ATPase activity than do slow-contracting (ST) red muscle fibers. However, it now seems that the situation is not quite this simple; the myosin ATPase activity is not simply either fast or slow, and grades of fastness or slowness among the FT and ST fibers may vary. Thus, the ST fibers of some athletes may have contraction speeds that approach those normally found in FT fibers.

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