Figure 21:

Different DIAm motor unit types are distinguished by their intrinsic, mechanical and fatigue properties and are classified as type S, FR, FInt and FF (A). Within a particular motor unit, all muscle fibers are homogeneous, as evidenced by myosin heavy chain (MyHC) expression (B). In the DIAm of most species, type I and IIa diaphragm muscle fibers have smaller cross-sectional areas than those of type IIx and IIb fibers (C). Differences in specific force between different fiber types is related to the different MyHC content per half sarcomere and differing unitary forces produced by different MyHC isoforms (D). Adapted from elements within refs 186 and 223.