A single muscle has to perform different functions, some more dynamic, which require speed and strength, and some more static, which require endurance. A single skeletal muscle contains different fibers for different speeds of contraction and different amounts of strength: slow-twitch (type I) fibers and fast-twitch (type II) fibers. Slow-twitch fibers take approximately 110 milliseconds to reach peak tension when stimulated. Fast-twitch fibers can reach peak tension in about 50 milliseconds.
Although only one form of type I fiber has been identified, type II fibers can be further classified into two major forms: fast-twitch type a (type IIa) and fast-twitch type x (type IIx). A third subtype of fast-twitch fibers has also been identified as type IIc.
The difference between types IIa, IIx, and IIc is not fully understood. Type I fibers are more frequently recruited than type IIa and type IIx fibers, and type IIc are the least often used. Most muscles are composed of approximately 50% type I fibers and 25% type IIa fibers. The remaining 25% are mostly type IIx; type IIc fibers make up only 1% to 3%. Current knowledge about type IIc fibers is very limited. The exact percentage of each of these fiber types varies greatly between muscles and also between individuals, especially among athletes in different sports. Table 5-1 is based on the current knowledge of these fibers.
As mentioned earlier, the percentages of type I and type II fibers are not the same in all the muscles of the body. The arm and leg muscles usually have similar compositions of fiber types. People with a predominance of type I fibers in their leg muscles probably have a high percentage of type I fibers in their arm muscles as well. A similar relationship exists for type II fibers. The soleus muscle is the exception: It is composed of a very high percentage of type I fibers in everyone.
Different sporting or other activities require different muscular functions, which make use of different ratios of type I and type II fibers.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.