Human skeletal muscle fiber types can be di- vided into three classes of fibers based on their biochemical and contractile properties. Two categories of fast fibers exist: type IIx and type Ha. One type of slow fiber exists: type I.
The biochemical and contractile properties characteristic of all muscle fiber types are sum- marized in Table 8.1.
Although classifying skeletal muscle fibers into three general groups is a convenient system to Study the properties of muscle fibers, it is important to appreciate that human skeletal muscle fibers exhibit a wide range of contractile and biochemical properties. That is, the biochemical and contractile properties of type 11x, type 11a, and type I fibers represent a- .continuum instead of three neat packages.